Articles – Bison Ops

6.8 SPC – 6.8 Western? Coming Soon!

February 18, 2023February 18, 2023 by Ben

Bison Armory has been making barrels for subsonic 6.8 SPC use for many years now. Ideal bullets for subsonic 6.8 SPC are 180 to 200 grains and are hard to find. The market for these bullets has been restricted to 6.8 SPC Subsonic ammunition in conjunction with 1:7 twist barrels. This market by itself has had limited growth due to the lack of projectiles available because the market is small. This is a classic catch-22 for anyone who wants to hunt and shoot subsonic with the 6.8 SPC.

The introduction of the 6.8 Western in 2021 has made it possible for things to change. With 1:8 and 1:7.5 twist barrels, and case capacity around 75 grains H2O, the 180 and 200 grain .277 caliber bullets have another market. I was able to test fire the 6.8 Western with 200 grain Woodleigh bullets recently and the results were exellent.

Shot from a Browning X-Bolt, the ammunition is capable of 1-MOA performance. At a muzzle velocity close to 2500 fps, the 200 grain bullet has 2775 ft-lb of muzzle energy. Excellent for deer and Elk hunting.

These are long bullets with a substantial bearing surface. The rifle just came in this week and is barely broken in. The barrel is fairly light weight compared with target barrels, yet it held up under shooting and produced good groups.

First group was shot on large highpower target as I didn’t have a 100 yard zero for the new rifle yet. This load was 48 grains of Accurate 4350 loaded to 2.745 OAL with Winchester brass and Remington 9-1/2 primers.

Next up 48.9 grains of Accurate 4350. Gordons Reloading Tool predicts 2477 fps with this combination. The single lone round on the upper left of the target is a 48.5 grain load to check pressure as I pushed the numbers up. The fired capacity of the cases measured 77 grains of H2O. Next time I’ll bring a Lab Radar and confirm these numbers, as well as run the round out to 300 yards. GRT puts this load at 49.8 ksi

We should have the particulars on these bullets from Woodleigh soon. Best guess is that the bullets will be available in 6 to 12 months, given the time required for manufacturing and import from Australia.

New SW Precision 224 Valkyrie loads

May 8, 2022May 8, 2022 by Ben

Given the scarcity of smokeless rifle cartridge propellant, I think a lot of us have been motivated to try different things. Over the course of the last two years I managed to get a few pounds of Shooters World Precision Rifle powder. Having sat on this powder for some time I recently had a chance to try it with something I hadn’t planned on: the .224 Valkyrie. I have a Palma match coming up at the end of May and decided to give SWP a try in order to preserve my other propellants and to see how it would do.

Quickload predicted excellent muzzle velocity and safe pressures in my Starline cases, that averaged 32 grains of H2O capacity after being fired but not resized. A load of 24 grains under a 90 grain Sierra MatchKing bullet was predicted to give 2700 fps from a 26″ barrel with the rounds loaded at 2.36″ OAL for single round feeding during the Palma match, and 24.5 grains should give 2750 fps at 96% fill and almost 100% propellant burn. Not bad!

All loaded up and ready to go, I benched the 26″ rifle that I built. It has no gas system and I run it as a straight-pull bolt action using a side charge upper receiver and bolt carrier. A 3D printed handle helps work the action.

Right out of the gate, two things were apparent: This ammo has excellent accuracy, but velocity is well below prediction. The 24 grain load ran 2609 fps average and the 24.5 grain load ran at 2650. However this ammo was shooting 1/3 minute groups without trying very hard. I turned to Gordons Reloading Tool because it has a feature that will tune a powder based on real world results. In this case GRT predicted that 24.9 grains of SWP would result in 2694 fps at the same ambient temperature as the previous shooting session. So with the new ammo loaded up, off to the range.

Match rifle with 26″ 224 Valkyrie barrel for upcoming 1000 yard Palma match. Bench target on left was shot at 200 yards. Reduced MR Target was shot prone with a sling at 300 yards. Shoot-n-See is about the width of the 10 ring and the SNS 9 ring is about the width of the X ring of the reduced MR target.

In this case the ambient temperature was about 10 degrees warmer than the last shooting session, and GRT provides a field to account for this difference. The updated prediction was 2700 fps, for a difference of 6 fps. Shooting the ammo on a Lab Radar gave 2715 fps which is close enough in my books! Best of all, the accuracy remained excellent.

I shot on MR targets that were reduced to shoot from 600 yards down to 300 yards, which is the maximum distance at my rifle club. I shot many more X’s than 10’s and I think I’m ready for the Palma match. While the velocity is nothing to write home about for a 1000 yard match, I think the accuracy will play a more important roll, and the velocity is sufficient that all I need to do is make good wind calls in order to shoot well. I’ll report with range results after the match at the end of May.

The 6.8 Bison Chamber

November 27, 2021November 26, 2021 by Ben

There are several popular chamber designs used in rifle barrels made to shoot the 6.8 SPC cartridge. The primary difference between these chambers is the freebore.

The original SAAMI chamber, known simply as the 6.8 SPC chamber, has 0.050″ freebore, while the current de facto standard chamber has 0.100″ of freebore. All of the chambers designed after the original 6.8 SPC chamber have increased freebore compared to the original. The 6.8 Bison chamber has 0.072″ of freebore in its design.

Note that the design freebore in a rifle chamber is the minimum specification. The dimensions in a rifle chamber can be anywhere from minimum (i.e. maximum material condition – when the least material has been removed from the barrel during chambering ) to maximum, which is the minimum plus the allowable tolerance (i.e. minimum material condition – when the most material has been removed from the barrel during chambering). Minimum and maximum material condition are not necessarily intuitive, and often seem backwards for an internal void machined into a piece of metal.

All else being equal, differences in freebore primarily affect the jump of a bullet to the lands. For a given cartridge, bullet jump to the lands depends on many factors, including the bullet shape and bullet seating depth (i.e. cartridge overall length or OAL), and the exact dimensions of the chamber in which the cartridge is fired.

For the 6.8 SPC cartridge, the difference in bullet jump between a minimum chamber and a maximum chamber depends on the leade angle, which is typically 1.5 degrees. The freebore DIAMETER varies from a minimum of 0.2781″ to a maximum diameter of 0.2801″. The point at which a given bullet will contact the lands of the rifling in the bore will move down the bore a distance of 0.001″ * tangent (1.5 degrees) = 0.038″ when changing from minimum diameter to maximum diameter freebore.

Selecting a rifle barrel from the Bison Armory inventory, with a 6.8 Bison chamber, I find the following OAL to bullet contact with the rifling for a selection of common 6.8 caliber bullets:

Bullet	OAL to lands	Bullet jump from 2.295″ OAL	Bullet jump – minimum spec	Bullet jump – SAAMI 6.8 SPC min spec
115 SMK	2.369	0.074	0.036	– 0.014
120 SST	2.390	0.095	0.057	+0.007
110 AB	2.408	0.113	0.075	+ 0.025
110 PH	2.405	0.110	0.72	+ 0.022
110 Hornady BTHP	2.380	0.085	0.085	– 0.003

OAL to lands for several popular 6.8mm bullets. Assumed magazine length 2.295″

I have assumed the maximum realistic magazine length, using PRI 6.8 SPC magazines, of 2.295″ to compare bullet jump. This chamber is definitely not minimum spec, virtually no chambers are because the reamers are always made to somewhere in the middle of the spec to account for reamer wear, plus some additional margin. The table indicates what the bullet jump would be for the minimum spec chamber ASSUMING the chamber used for the measurements is a maximum spec chamber, in order to have the most conservative results possible.

The final column indicates what the bullet jump would be if the measured chamber was maximum spec and we were to compare with a minimum spec SAAMI 6.8 SPC chamber. In this case it’s possible that we would be jamming the bullet significantly into the lands when using the 115 SMK and 110 Hornady BTHP bullets (indicated by (-) sign on the delta).

The takeaway from this table is that the 6.8 Bison chamber provides ample bullet jump to the lands for 6.8 SPC rifle cartridges loaded to an OAL that will fit maximum magazine length of 2.295″. This is the primary requirement for safe operating pressure. Jamming a bullet into the lands, or loading with little jump to the lands, is known to increase maximum pressure significantly. By loading at least 10 to 20 thou (0.010″ to 0.020″) off the lands, we ensure that pressures will not spike when the ammunition is fired in the rifle.

In general, the 6.8 Bison chamber provides substantial bullet jump to the lands for magazine length loaded ammunition. The popular 6.8 SPC II chamber exceeds the 6.8 Bison chamber bullet jump, all else being equal, by an additional 0.028″. This seems needlessly excessive to me and excessive bullet jump is known to be detrimental to accuracy for many or most bullet and cartridge combinations, with a few exceptions (e.g. some Burger VLD bullets in some calibers appear to shoot with better accuracy when jumped between 80 to 100 thou to the lands).

There are many other factors that influence chamber pressure for a given cartridge and load, and freebore is only one of them. The capacity of rifle cases varies between lots and manufacturers, and can have a very significant impact on load pressure. The many factors that influence chamber pressure is the main reason hand loads must always be worked up whenever new load parameters are introduced. Chamber design and allowable tolerances is a primary reason why ammunition manufacturers load on the light side – their ammunition must be safe to shoot in all chambers that meet SAAMI specifications.

Case Volume and Loading for Long Range Precision: Part 2 Case Study

October 4, 2021April 27, 2021 by Ben

In preparation for an upcoming 1300 aggregate Palma match to be fired at 800, 900, and 1000 yards over two days, I have measured the volume of 200 Winchester 223 cases that are otherwise ready to shoot. These cases have a mean as well as median capacity of 30.4 in grains of H2O (from here forward I will refer to grains of H2O as simply “grains”). The standard deviation was 0.12 grains, with a minimum of 30.13 and maximum of 30.88. With the expectation of 20 to 35 fps muzzle velocity per grain variation in case capacity, I expected the velocity variation to be between roughly 15 and 25 fps between the cases near the min and those near the max.

Ordering the cases from low to high volume gives the following distribution:

The majority of cases are between 30.2 and 30.55 grains with tails beyond those thresholds. I removed all these cases in the tails from the batch that I will be shooting at the upcoming match.

These rounds will be loaded with Hornady 90 grain A-Tip bullets to an OAL of approximately 2.45 inches. I expect to reach approximately 2600 fps with this ammunition at the muzzle.

For this study, I took the high and low six cases at each end of the distribution to measure velocity for comparison. My plan is two-fold, two see if these outliers (using this term loosely) can significantly effect my scores at 600 to 1000 yards, and if so, by how much. This is a small sample size as I want to keep the cases that are not at the tail ends of the distribution for the upcoming competition.

After shooting the 12 rounds I found that the muzzle velocity of the high-volume cases was 2581 fps with SD of 9.7 fps and the muzzle velocity of the low-volume cases was 2606 fps with SD of 14.9 fps. This gives a total spread of 25 fps, at the high end of the expected range.

Notice how the high capacity cases have LOWER velocity than the low capacity cases. This reflects the higher pressure generated by equal powder charge weights in a smaller volume, and is in agreement qualitatively with Quickload as well as general experience.

Running this data through a custom ballistics calculator that uses the mean velocity to get a fixed launch angle for all rounds gives the following vertical variation at the target for distances between 200 and 1000 yards:

Will this humble 223 load be capable of 1000 yard performance? Here’s how the bullet speed drops with distance for this load using Hornady’s G1 BC of 0.585.

Taking the rule of thumb for the transonic limit of 1.2 Mach, in which we select a Mach 1 value of 1125 fps corresponding to air at 68 degrees F which gives a transonic limit of 1350 fps. Our ammunition comes out right at the limit, and its performance has been proven in competition as well.

In my experience a bullet need not be going too fast at the target to perform with good accuracy. Provided the bullet is properly stabilized it will continue to exhibit good accuracy to Mach 1.1 and potentially lower. It is running out of gas for sure at Mach 1.2 though so that is a good speed to aim for at the target.

So what does this all really mean for a competitive target shooter or anyone else who wants to place rounds accurately at long range? Let’s start at the low end of long range, 600 yards, usually referred to as mid-range by high power competitors. Here is where the rubber hits the road.

Suppose a sling shooter and his weapon are able, all things being equal, to hold the 10 ring most of the time at 600 yards and 1000 yards. We can simulate such a shooter, who would have a mean radius right around 0.9 (see ballistipedia.com for a proper explanation of mean radius). We can also neglect wind for this study but add in the vertical variation due to variation in muzzle velocity. At 600 yards, how many points and X’s would this shooter give up due to high or low capacity cases? Let’s have a look at the extreme example in which the shooter only shoots the cases at the tails of the distribution. Simulating 10 different groups at 600 yards we would have the perfect cases on the left and the tail-capacity cases on the right.

Note: the inner circle is the F-Class X ring. Clearly these are not F-Class groups being simulated and I will address the effect of muzzle velocity in competitive F-Class in an upcoming post.

We see our shooter dropping a couple points and X’s due to case capacity variation most of the time. But we know that most of our cases are going to be in the good portion of the distribution, not at the tail. So an outlier could cost us a point, but we see why top shooters who do not measure and account for case volume rarely drop points and keep on winning anyway. Thank goodness that shooting, especially from a sling, still comes down to shooting ability, especially the general fundamentals and wind reading. But still, a competitive match could still come down to one or two rounds with relatively high or low muzzle velocity.

At 1000 yards the situation is a little bit more dramatic, but still we’re talking a few points or X’s and this for all cases at extreme ends of the distribution:

I find it amusing that sometimes inaccuracy in the weapon and ammunition can work to our favor. If we throw a shot high that would otherwise have been low due to other random variation in the weapon, wind, and ammunition, then we get lucky and save a point or two. But in the aggregate we cannot achieve winning scores consistently in this way.

So we see that muzzle velocity variation due to case capacity variation is another knob we can turn in our pursuit of perfection, but we still have to be good at shooting to reach our full potential and win matches.

Case Volume and Loading for Long Range Precision: Part 1 The Basics

April 9, 2021April 9, 2021 by Ben

All long range competitive shooters agree that ammunition must be accurate at short range in order to be accurate at long range. They also know that this is a necessary condition, but not a sufficient one. In addition, the ammunition should have very low variation in muzzle velocity and the bullets should have a good ballistic coefficient.

Many factors influence muzzle velocity, and variation in these factors will lead commensurate variation in muzzle velocity. These include:

Powder charge
Case neck tension
Case mouth uniformity
Flash hole uniformity
Primer consistency
Bullet weight
Bullet seating depth
Case volume
Recoil technique (strange but true – a topic for another post)

Each of these factors has a limit to which we can minimize variation, and at some point the effort to decrease variation leads to diminishing returns. In this article I will consider ammunition that is between reasonably and superbly controlled for most of the factors in the list and both how and how much controlling additionally for case volume can result in improved variation in muzzle velocity.

Suppose Case Volume was the Only Factor

The simplest place to start is with ammunition that is perfect in every way except for variation in case volume. Quickload and experiment have both shown that for many typical cartridges, powders, bullets, etc., muzzle velocity varies with case volume at a rate of approximately 20 fps to 30 fps per grain of H2O as a measure of case capacity. From here I will use “grains” in place of “grains H2O”, the “H2O” being implied.

Consider ammunition from a simulated population of 500 284 Win cases in which the volume of the cases is normally distributed about a mean of 69.1 grains with a standard deviation of 0.175 grains. I got the mean and SD used here from real world measurement of 100 cases. Assuming that a case with 69.1 grains capacity produces 2800 fps muzzle velocity for a 180 grain Berger Hybrid, and a 20 fps per grain volume variation, a randomly generated population is shown in the following graph:

Most of the muzzle velocities are centered around 2800 fps as expected and we see a high an extreme spread of about 20 fps, which is not surprising since the population of cases has an extreme spread of case volumes that is about 1 grain.

So what does this mean in practice? How will this otherwise perfect lot of ammunition shoot, all else being perfect? Using a typical G1 drag model for the Berger 180 Hybrid we get the following vertical distribution on paper from 600 to 1000 yards down range:

And again with vertical dispersion measured in minutes of angle instead of inches:

I prefer looking at the plot that shows POI vertical variation in minutes because it is easier to relate to score in high power rifle competition. At 200 yards the vertical variation is very small at just over +/- 0.1 minutes. I don’t know many people outside short range bench rest who would lose too much sleep over that. At 600 yards we’re approaching +/- 0.5 minutes, now that’s something. In Service rifle that can cost you an X or a point, and in F-Class we are definitely talking points to lose for a shot that would otherwise be near the inside edge of scoring a 10, and at 600 yards these days, X-count often separates winners and losers.

For those of us who shoot long range matches, which are typically from 800 to 1000 yards and shot on an LR or LR-F target, you can see that we’re talking serious points to lose with the vertical spread approaching +/- 1 moa.

Adding Measurement Error

We cannot measure case capacity perfectly. How much does measurement error influence our results so far? With the Bison Armory Case Capacity Gauge, I have performed some experiments to determine measurement error and a normal distribution with zero mean and standard deviation of 0.025 grains fits the data. Adding random measurement error to the muzzle velocity vs case capacity shown earlier, in which we keep the muzzle velocity where it was for the perfect case and only vary the measurement due to error we get the following:

I think it is clear from this figure that case capacity measurement error is not a significant factor. If case volume was the only contributor to muzzle velocity variation, and we can measure case capacity as accurately as indicated in the figure, then it would be a simple thing to produce a batch of ammunition for a match that had minimal velocity spread.

Of course it’s not so simple. In the next post I’ll add muzzle velocity variation from all other sources and we’ll see how that complicates matters, and also how to make the best use of case capacity measurements to decrease muzzle velocity dispersion.

308 Win Case Capacity and Muzzle Velocity

April 6, 2021April 5, 2021 by Ben

I conducted a brief study of case capacity and its effect on muzzle velocity this weekend. Such studies are easy because spending time at the rifle range is fun. They are also difficult because time is limited and it takes a lot of trigger time to get statistically significant results.

This study is not rigorous in that insufficient data was collected to prove any correlation between muzzle velocity and case capacity for a given brand of case, but enough data was collected to show a link over several brands of cases. The difficulty here is that there is more to muzzle velocity variation than case volume, but if the variation in capacity is great enough, we see the effects clearly.

Starting with the case capacity in grains of H2O between a selection of new and once fired 308 Win brass from Lapua (once fired), Federal (once fired), SSA (new), Winchester (new), and Hornady (once fired).

SSA has the lowest capacity while Hornady and Winchester were about the same at the highest capacity. Approximately 2 grains of H2O capacity separate the lowest from the highest. We expect that all else being equal (i.e. the same powder charge and bullet weight etc.), the cases with the lowest capacity will exhibit the highest muzzle velocity and vice-versa. Here’s the results from the range session shooting off-hand with an M14 (shot pretty well, one 10-round string was 96-2x)

In this figure clear correlation between case volume and muzzle velocity is apparent. Obviously other factors influence muzzle velocity besides case volume as there is significant variation in muzzle velocity that does not correlate with case volume. For example, the SSA brass (grey dots) has lower muzzle velocity than the Federal brass (orange dots) even though it clearly has lower case volume, which generally correlates with higher muzzle velocity.

Given that the powder charges were thrown by an Autotrickler to 41.2 +/- 0.02 grains of H4895, the powder charge is the most consistent thing besides bullet weight at 168 grains for the Sierra Match King bullets used in this test. Notice also that for each 10-shot group except for the group shot with Hornady brass, the variation among the group does not correlate much at all with case volume. This is to be expected with sample sizes this small. Even so, the correlation among the data in general agrees with the prediction made by Quickload between 20 and 30 fps per grain of case volume, all else being equal.

The Hornady brass did show good correlation between case volume and muzzle velocity so let us consider it more closely.

This correlates with the prediction given by Quickload but is still too small a data sample to be taken as strong evidence. And there lies the problem as always with load development and accuracy: the difficulty with which we obtain meaningful results due to the constraints involved in gathering statistically significant data. Barrels heat up, fatigue sets in, Lab Radars fail to register a shot, and so on.

Ideally I would turn necks and be very careful about neck tension, flash holes, and the rest, and then shoot 50 to 100 rounds of each brand case. I’ve also found that correlation is stronger if the volume of the fired case is measured before resizing and compared with the muzzle velocity from the previously fired shot.

So take the data as it is, a point from which we can move forward, no more, no less, and an indication that what we expect is true, so now we have to be more careful to prove it.

In an upcoming article I will discuss strategies for using case volume measurements to inform load development for match shooting at 600 yards and beyond.

Measuring Cartridge Case Volume

March 18, 2021March 18, 2021 by Ben

In this post I address the use cases for measuring case volume. Reloaders have gotten by for quite a while without measuring the volume of every case. Most reloaders never measure case volume. What are the reasons anyone would want to?

If you are interested in the Bison Armory Case Volumizer you can see them in our online store here.

In the past, measuring case volume was a slow task. Typically the reloader would weigh a case, fill it with water, then re-weigh the case to measure the weight of the water that filled the case. Obviously not the most desirable method. With the new Bison Armory case volumizer, the task is simplified to the point that it takes only minutes to accurately measure the volume of 100 or more cases.

Prior to this, there was not much point in discussing the reasons for measuring case volume. The cost in terms of time and effort were simply not worth the resulting information. Now that cases are easily measured with the Bison Case Volumizer (BCV), the question of why becomes interesting.

Checking for bad cases

Split case necks and other defects are real. Are you hunting? Shooting in a match? Going to a training class? The BCV easily detects any case with split neck or other compromise to its structure. A volumized case is one you can rely on.

Pushing the limits

Bison Armory does not advise pushing muzzle velocity to the limits, but we know some reloaders will do this. Suppose you are reloading all Winchester cases and a Starline case sneaks in. If you are pushing velocity to its maximum safe limits, a case with lower capacity than expected, like you might get from one from another brand sneaking into your batch unknown, could cause catastrophe. The BCV will detect these cases. In addition, suppose a new lot from the same manufacturer happens to be low. Manufacturing tolerances will vary somewhat even for the best manufacturers. The BCV when used properly and within its limitations, will alert the reloader to these sort of situations.

Long range accuracy

At 500 yards and beyond, variation in muzzle velocity starts to have a significant effect on accuracy. I shoot long range matches in F-Class and Service Rifle categories. Pushing the 223 Rem to 1000 yards is a lot of fun with the right bullets, but how much does variation in case volume affect long range accuracy? Quickload is a handy tool for cursory investigations into this question.

We can start with the common question of how much does variation in powder charge affect velocity and hence vertical dispersion at long range. For the 223 Rem with my personal load of 22.2 grains of H4895 in a Winchester case behind a 90 grain Sierra MatchKing bullet, we find a nominal muzzle velocity of 2550 fps. Quickload says +/- 0.1 grains of H4895 will result in +/- 10 fps out the muzzle. For my pet 223 long range load, that means the following vertical dispersion at distance:

Distance (yards)	Velocity Low/High (fps)	Drop Low/High (in)	Drop Low/High (moa)
600	1718 / 1734	86.9 / 85.2	13.8 / 13.6
700	1601 / 1617	134.6 / 132	18.4 / 18
800	1492 / 1506	195.8 / 192.1	23.4 / 22.9
900	1390 / 1404	272.8 / 267.6	28.9 / 28.4
1000	1298 / 1310	367.7 / 360.7	35.1 / 34.4

Now we know why long range shooters spend $1000 on an Autotrickler powder measure in order to throw charges quickly to +/- 0.02 grains. 1.7 inches at 600 yards and 7.0 inches at 1000 yards will lose you some X’s and 10’s.

What about case volume variation? Quickload tells us that variation of +/- 0.25 grains of powder will result in a muzzle velocity spread of 20 to 30 fps in the 223 Rem and variation of +/- 0.5 grains in the 260 Rem will see about 20 to 30 fps variation as well, depending on bullet, powder, and powder charge etc. As a fraction of case volume the variation is about the same.

Note: The velocity change for 223 Rem from +/- 0.25 grains of case volume is about the same as for +/- 0.1 grains of charge weight. So if you care about charge weight variation you probably ought to at least be interested in case volume variation.

I have verified this through experiment. Admittedly not a huge numbers on the surface, but how will this affect my performance in a match? With a low muzzle velocity of 2546 and a high of 2563 (difference of only 18 fps) we get the following trajectory table using Hornady’s ballistics calculator:

Distance (yards)	Velocity Low (fps)	Velocity High (fps)	Drop Low (in)	Drop High (in)	Diff (in)	Drop Low (moa)	Drop High (moa)	Diff (moa)
500	1847	1861	50.6	49.9	0.7	9.7	9.5	0.2
600	1723	1736	86.3	85	1.3	13.7	13.5	0.2
700	1606	1619	133.7	131.7	2.0	18.2	18	0.2
800	1497	1508	194.6	191.6	3.0	23.2	22.9	0.3
900	1395	1406	271	266.9	4.1	28.7	28.3	0.4
1000	1302	1312	365.3	359.7	5.6	34.9	34.4	0.5

The X-ring of the MR target is 3 inches and the 10 ring radius is 6 inches. At 500 yards the 0.7 in difference between high and low is pretty small but could cost an X or a 10 on shots that the shooter puts at the outside of the ring. At 600 yards the variation almost doubles and can start costing X’s and points.

For 800 to 1000 yards we shoot at the NRA LR target with an X-Ring that is 5 inches in radius and a 10-Ring that is double with a 10 inch radius. It is clear that the difference of 3, 4.1, and 5.6 inches between the low and high velocity values at 800, 900, and 1000 yards respectively can cost a lot of X’s and points. At 1000 yards in particular, the vertical dispersion is slightly larger than the width between rings.

Volume variation in Winchester 223 brass

I measured the volume of 98 Winchester 223 Rem brass cases and got the following results

The low value was 30.31 grains H2O and the high value was 30.73 for an extreme spread of 0.41 grains with a mean of 30.56, a median of 30.57, and a standard deviation of 0.09 gr H2O. Pretty good results actually. I’ve seen outliers with much bigger deviations. This is good brass. An outlier will definitely cost points during a match.

Once measured, what do you do with the cases? My personal approach is to omit any outliers and then split the rest at the mean or median to use for a 20 round match plus sighting shots. In this situation they are effectively the same. In this way I assure that my ammunition for a 20 round match will exhibit minimal vertical dispersion at long range, having in this instance a variation in case volume of +/- 0.1 grains H2O

In the next article I will compare measuring case volume by water weight using an FX-120i scale with the results from the Bison Armory Case Volumizer.

Reduced NRA High Power Target Dimension Calculator

February 13, 2020February 13, 2020 by Ben

I’ve created a calculator that computes reduced NRA High Power target dimensions so that you can create your own SR, SR-3, MR-1 and other targets to match your distance and caliber:

ReducedTargetDimensionsWorksheet Download

To draw the targets you will need a vector graphics program like Corel Draw. They have a home and student version available here:

https://www.coreldraw.com/en/product/home-student/

Here’s an example of a target I created for shooting offhand at the SR 200 yard target at our local 25 yard indoor range:

SR-Target 25YD Reduced Download

The target is made to be printed on tabloid sized 11×17 paper. The HP 7740 wide format printer for around $180 can’t be beat and has printed many of these targets for me.

Rifle Ammunition Load Workup

August 17, 2019August 17, 2019 by Ben

We have all read forum posts in which the author describes their load workup process and the resulting shot groups. Many of us have written such forum posts, myself included. Typically these posts start with a detailed description of the rifle and ammunition components and then outline the loading process and strategy. Factors such as cartridge OAL, powder type and charge weight, and bullet will be varied to some degree, and the author will shoot several five-shot groups in which each is a little different from the others.

Here’s a good example of such a post on the 224 Valkyrie Forum.

When the shooting is done, we get to see the targets and corresponding shot groups from all the lead that was sent down range. The five-shot groups are compared, and conclusions are arrived at that feel good and seem reasonable. Usually one or two groups stand out and the author declares that the weapon really liked that particular variation of ammunition, and the less desirable groups are thought to contain fliers from poor shooting technique or a larger group dispersion from an ammo variation that the weapon didn’t like.

I am just as guilty as the next shooter of going about load development in the manner described above. I’ve been looking into the random nature of ballistics a lot lately, mostly driven by the content at Ballistipedia, and the more I read up on the subject, and the more I apply the statistical analysis to my shooting data, the more I see that this typical ammunition loading development process is extremely bad.

Thanks to computers and math, I can demonstrate why this common load development process is so terrible. Consider the following simulated load development results. The center of the black and purple circles is the true point of aim and the black dashed circle represents 1-moa diameter:

Simulated group 1 – dashed black line is a 1-moa diameter circle

In the manner of our typical rifle load developer (self included) I would conclude with the following: Group 1 is a good start. Group 2 is headed in the right direction and if my breathing had been better I’m pretty sure it would be a tighter group. Group 3 not only did the rifle did not like this load, but I flinched and got that flier you see. Finally, for Group 4 the rifle really liked this load. So what’s the problem? Don’t those conclusions seem to follow from the data.

The problem is that there’s absolutely ZERO difference between the four groups shown above. Sure, the bullet holes are in different places in the four groups, but the exact same statistical distribution generated all four groups. In this case they are generated from a distribution that has a mean radius of 0.4 inches. The purple discs indicate the expected extreme spread that would be generated by a weapon and ammunition with that shoots with a 0.4 inch mean radius. The inner disc represents the largest expected extreme spread of the smallest 10% of groups that this distribution would generate, while the outer disc represents the smallest expected size of the largest 90% of extreme spreads that would be generated. And that’s knowing ahead of time that the rifle will shoot with 0.4 inch mean radius.

This result demonstrates one thing clearly: Much time and effort is wasted in load development. The rifle shooter who is interested in precision load development must pause for a moment and ask this question: What is the purpose of my load development process? It could be for one of the following:

Hunting
Service rifle competition
F-class competition
CQB defense
Other?

If hunting is the purpose for the load, then the hunter would likely prefer a flat shooting load that is accurate enough for the job and delivers adequate energy at the point of impact for a clean kill. Hunting range may vary depending on the game, environment, and other circumstances.

For competition shooters, accuracy is the primary consideration, but muzzle velocity influences time of flight and hence the variation due to wind. Variation in muzzle velocity will also have a significant effect at longer ranges on both vertical dispersion in general and horizontal dispersion due to wind.

Let’s consider the case of a 600 yard prone match for F-class or service rifle. In this case, we would like adequate muzzle velocity and then the best accuracy possible from our ammunition. Suppose this rifle is capable of a mean radius of 0.25 inches. A 5-shot group from this rifle might look like the following:

Typical group 0.25″ 100-yard mean radius – dashed black line represents 1-moa diameter

Will this group suffice for our upcoming 600–yard prone match? It’s pretty tight, but notice that it is off-center by enough that if we zero our rifle from this group, we will score poorly and not place very high in the match. If we shoot a 10-shot group we’ll get something like the following:

10-shot group with 0.25″ 100-yard mean radius

Ten shots does a little better. The purple discs that show expected extreme spreads are getting closer together, and our zero from this group would do for competition. Still, we could shoot several of these groups and might be tempted as we were with our 5-shot groups to come to invalid conclusions. Another 10-shot group could be:

Another 10-shot group with 0.25″ 100-yard mean radius

This group is tighter than the last one but the zero is misleading and a scope zeroed off this group will lose us points in our match. How does a 20-shot group look?

20-shot group with 0.25″ 100-yard mean radius

This looks like a group that we could potentially start to trust to zero our rifle and to be representative of the potential of the rifle and ammo that we are shooting. It has an expected mean radius of .24″ and the actual group mean radius measures 0.25″. They expected extreme spread 90% min and max circles are coming closer together as well and will converge as the number of shots in the group goes up. They don’t really converge until the round count is in the hundreds, which is impractical.

The point of this post is to show that 5-shot groups cannot distinguish in any meaningful way between groups that are shot with small variations in loading parameters. A 10 shot group might start to give some sort of confidence, but 20 or more are needed to at least establish a baseline. The real question is how to tell if a given group is statistically different from the baseline? How many shots are needed and how different to the groups need to be in order for a claim to be made that some change in loading parameters was a likely cause of the difference? I will try to address that in a follow-up post.

6.8 SPC Not So Special, eh?

November 4, 2018November 2, 2018 by Ben

On April 4, 2015, the Firearm Blog’s Nathaniel F Posted a very interesting article titled “Not So Special: A Critical View Of The 6.8mm SPC“. The article presents a critical review of the 6.8mm SPC cartridge and an extensive comparison with both typical and high performance .223 / 5.56 NATO ammunition. The results of the comparison indicate that the 6.8mm SPC cartridge cedes too much in terms of trajectory and velocity to modern offerings in 5.56 NATO to make up for what it gains in projectile energy at range. In summary, Nathaniel F finds that the 6.8mm SPC is useful for medium game hunting at modest ranges, and not much else.

At Bison Armory, we have a special affinity for the 6.8mm SPC cartridge, and so I have studied Nathaniel F’s article and I present here a critical response to his excellent and generally well considered article.

For the purposes of this review of Nathaniel F’s article, I will only consider factory ammunition and not hand loaded ammunition to simplify the discussion.

Historical Review

Nathaniel F’s article (I’ll refer to it going forward as “NFA”) begins with a brief history of the development of the 6.8mm SPC cartridge (6.8 SPC). This historical review is excellent and I have nothing more to write about it, except to say that if you are interested in the 6.8 SPC, this historical review, with links, is a good place to start.

6.8 SPC case geometry and projectiles

NFA provides good information regarding the case information including parent case, case taper, shoulder angle, case length, and case capacity. No quibbles here. NFA then mentions projectile selection:

It uses standard .277″ diameter projectiles, although it is limited in its selection of those by the overall length. In general, though the projectile diameter is the same, the 6.8mm Remington requires totally new projectiles versus existing .270 caliber rounds

This is not entirely true. Existing .270 caliber rounds, such as the 130 grain Berger VLD and 130 grain Woodleigh PPT are suitable for hand loading. But in general, this claim is fair.

Further:

It is very curious to me that a round that was designed with an accurized designated marksman’s rifle in mind also features such a short ogive and low muzzle velocity. Typically, medium/long range precision weapons platforms place a premium on both the ballistic coefficient and velocity of the projectile, seeking to obtain as laser-like a trajectory and as high a retention of energy as possible. The 6.8mm SPC runs directly counter to this.

And:

In light of this, the 6.8mm SPC’s maximum ogive length is the first curiosity of the cartridge’s design. With only 2.07 calibers available space for the ogive, the maximum ogive length for the 6.8 SPC is less even than that of the 5.56x45mm or 7.62x39mm cartridges.

Here’s where I begin to quibble with Nathaniel F’s piece. Does he mean the maximum length in terms of calibers or inches? The .223 Rem / 5.56 NATO has a 0.5″ of space available for the ogive, which is 2.23 calibers, while the 6.8 SPC has 0.574″ or 2.07 calibers. This presumes loading to SAAMI / NATO specifications which limit the OAL of both cartridges to 2.26″. Next:

This is very short for a rifle cartridge designed for maximum performance out to 450 meters, and limits the form factor of compatible secant-ogive projectiles to above 1.15 i7 (lower form factors yield higher ballistic coefficients), with tangent ogive projectiles having form factors as high as 1.32 i7. For comparison, the M855 projectile offers a 1.166 i7 form factor, despite not being particularly well streamlined. Finer projectiles compatible with the 5.56mm round offer i7 form factors as low as 1.09. Even when magazines allowing longer ogives are used – which it should be noted also would allow the use of longer and finer 5.56mm projectiles – the available relative space is still not significantly greater than that of the 5.56mm and 7.62x39mm cartridges. Further, as of yet there do not seem to be any manufacturers making .277″ projectiles with ogives designed for these longer magazines, and thus they only offer a velocity advantage to handloaders seating existing short-ogive bullets less deeply in the case.

This is a lot of blather. Form factor (or Coefficient of Form) is a very useful coefficient for describing the aerodynamic drag that acts on a bullet in flight, more useful in some ways than the ballistic coefficient. If you don’t understand the importance of form factor to ballistic performance, or how it relates to BC, I suggest a quick read of this article by Brian Litz at Berger Bullets. I think the previously quoted paragraph contains a lot of blather because statements like “This is very short for a rifle cartridge designed for maximum performance out to 450 meters,” are misleading. The idea of “maximum performance” is meaningless without providing a measure of optimality. This would in turn imply a cost function, and none is provided. This is unfortunate because the history of the 6.8 SPC provided near the beginning of the article clearly states the objectives of the cartridge program:

… to develop a new capability that would increase incapacitation, lethality, and range over the existing 5.45x39mm, PRC 5.8x39mm, 7.62x39mm and 5.56x45mm NATO cartridges.

…

Once all the performance data were compiled the team briefed the Commander on the results, and recommended that the 6.8mm [of the 6.5mm, 6.8mm, and 7mm projectiles considered to be combined with the 30 Rem parent case – ed.] provided the best overall terminal, reliability and accuracy performance out to 450 meters.

I don’t notice anywhere in the terse history provided in the article a requirement of a nebulous “maximum performance out to 450 yards.” I will put the actual statement of purpose to the test later on. So the loaded opening sentence about the purpose of the 6.8 SPC cartridge leads us into the rest of the paragraph which seems reasonable but is guilty of a serious error or omission of fact: The claim that projectiles for the 6.8 SPC are limited to form factors above 1.15 is simply false. This table summarizes form factors and ballistic coefficients for several 6.8 SPC specific projectiles and the 77 grain SMK for comparison:

Bullet	Weight (gn)	Diameter (in)	SD	G1 BC	G7 BC	i7
Sierra MatchKing	77	0.224	.219	.362	.185	1.184
Nosler Accubond	100	.277	.186	.323	.165	1.127
Nosler Accubond	110	.277	.205	.370	.191	1.073
Hornady BTHP	110	.277	.205	.360	.186	1.102
Sierra MatchKing	115	.277	.214	.324	.167	1.281
Hornady SST	120	.277	.223	.400	.207	1.077

77 grain SMK G1 = 0.362, G7 = 185, i7 = 1.184.

100 grain Nosler Accubond G1 = 0.323, G7 = 0.165, i7 = 1.127.

110 grain Nosler Accubond G1 = 0.370 , G7 = , i7 = 1.073.

110 grain Hornady BTHP G1 = 0.360, G7 = 0.186, i7 = 1.102.

115 SMK BTHP G1 = 0.324, G7 = 0.167, i7 = 1.281.

120 grain Hornady SST G1 = 0.400, G7 = 0.207, i7 = 1.077.

I don’t know where Nathanial F. got his data, but he needs to check his numbers. All of the above bullets were available and in use in commercially produced ammunition at the time of his writing from Silver State Armory, Hornady, and others.

Further, as of yet there do not seem to be any manufacturers making .277″ projectiles with ogives designed for these longer magazines, and thus they only offer a velocity advantage to handloaders seating existing short-ogive bullets less deeply in the case.

Several bullets are available that can be loaded to SAAMI case length or up to magazine length for the longer PRI, Magpul/LWRC, and C-Products magazines. Cavity Back Bullets has two different 120 grain bullets with high ballistic coefficients (.365 G1 hunting bullet and .400 G1 target/tactical bullet). The .400 G1 120 SST can be loaded long for increased muzzle velocity, as can the .370 G1 110 Accubond. Ammunition made with Berger 130 VLD bullets must be loaded to an OAL that requires the longer magazines. 120 to 130 grain bullets from Nosler, Cutting Edge Bullets, and others can be used as well.

Additional Metrics

The next few paragraphs find NF looking for any metric he can use to show that the 6.8 SPC doesn’t measure up to the 5.56×45 cartridge. Many of these numbers may be worth considering from a design point of view, but the 6.8 SPC is no longer in design phase; it is now an actual cartridge and there’s little point comparing design parameters when we can compare actual ballistics performance. Hence, I call foul.

I don’t plan to address the following information provided by NFA except to express my emotional displeasure with the topics:

Fineness ratio, bleh

Relative capacity, bleh

Powley computer bunk, bleh

The NFA article refers to MK262 running at 68kpsi, to which I says pardon? NFA refers to “more thermally stable propellants, but link is broken and what is he talking about? Another link to the material is here:

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2006/smallarms/faintich.pdf

Claims 2800fps from 18″ barrel, maybe on a hot day. Black Hills claims 2750fps and a recent trip to the range on a 50 F day found 2722 fps from an 18″ barrel and 2790 fps from a 20” barrel. So I find some exaggeration in the velocity claims made in the article.

NFA also makes up a metric called “internal specific energy” that is admittedly “not generally used in the relevant literature.” Bleh.

Bolt Thrust

NFA considers bolt thrust, because if he can find a problem with the 6.8 SPC he’s going to report it. He explains that the 5.56 NATO produces 5432 lbf bolt thrust while the 6.8 SPC produces 6537 lbf, and concludes:

The 6.8mm SPC round evidently puts much more stress on the rifle’s bolt than the 5.56mm round, which may cause the bolt to break sooner.

To which I reply: I’ve sold thousands of 6.8 SPC bolts, barrels, upper assemblies, and rifles, and I’ve yet to have a customer report a failure of a bolt that could be attributed to bolt thrust. Ugh. Again, real world vs design metrics crush his arguments. Naturally the 6.8 generates more bolt thrust than the .223 Rem because F = ma and K = 1/2mv^2, , i.e. physics, and the 6.8 SPC has significantly higher kinetic energy at the muzzle than the 5.56 NATO. The question is really: “Can AR-15 bolts manage the bolt thrust associated with the higher power cartridge?” The data I have available replies with an incontrovertible “yes.” The consideration of bolt thrust is interesting but really results in little difference between the two cartridges.

External Ballistics

For a proper comparison of the performance of the 5.56 NATO with the 6.8mm SPC, external ballistics are where the rubber meets the road. NFA begins with a comparison of one of the lamest 6.8 bullets from a terminal performance point of view, the Sierra 115 MatchKing. This bullet is very stubby, has a low G7 BC, and high form factor. This was one of the earliest 6.8 SPC specific bullets and it shows. Comparing this bullet to the 77 grain .224 SMK in the MK262 may seem like apples to apples, but it really is not.

NFA also compares Hornady BTHP bullets, specifically the 75 grain .224 vs the 110 grain .277. In this case the 6.8 fares somewhat better but the results presented indicate little advantage in retained energy of the 6.8 at range, and inferior trajectory and velocity. NFA calls the 6.8 SPC performance “Lackluster,” but is it really? He gives the 110 grain Hornady BTHP a BC of 1.8 when 1.86 is probably more accurate. He increases the velocity of the 6.8mm Hornady round to 2660 fps while also increasing the velocity of the 75 grain Hornady BTHP to 2840 fps. I think he is giving preference to the 5.56 NATO numbers here and not being as objective as he could be. The 75 grain Hornady would produce about 2837 fps if we equate energy of the 77 SMK with the 75 BTHP. However, my range testing, along with the stated muzzle velocity for an 18″ barrel for the MK262 from Black hills, indicate that 2750 fps is more realistic. Hence, a more realistic value for the 75 BTHP is 2785 fps.

NFA external ballistics analysis included comparison of velocity, drop, and energy. To that I am adding optimal game weight (OGW), which is a dubious metric if used in isolation, for projectiles of dissimilar size (i.e. pellets vs baseballs), and without consideration for bullet construction and purpose. However, OGW for comparison of similarly constructed bullets in a range of calibers in which the smallest is not less than 70% of the largest (e.g. comparing .224 to .308).

The above numbers give the following ballistics (drop, velocity, energy) for the 4 rounds:

Drop data indicates the Hornady 75 grain BTHP loaded to the same energy as the MK262 is the best load considered. As range increases past 600 yards the 75 BTHP is dominant. The Sierra 115 grain MatchKing is a complete slouch and highly suboptimal. While an accurate bullet for shooting the 6.8 SPC out to 300 yards, it doesn’t have much purpose beyond this use. The 110 grain Hornady BTHP is vastly superior to the 115 SMK beyond 300 yards.

Velocity data is even more stark than the drop data. The disparities at the muzzle carry to all ranges, with the 75 BTHP blowing all challengers away. The 110 BTHP catches up to the MK262 with the 77 grain SMK. In this case the 110 BTHP stays above what NFA calls the transonic limit of 1300 fps past 750 yards while the MK262 achieves only about 680 yards with this distinction.

The energy plots show the clear advantage of the 6.8 SPC at close range, and to some degree shore up NFA regarding retained energy at range. However, when we adjust the ballistic coefficient of the 110 BTHP, it is clearly superior at range to all the other offerings. The 115 SMK is again demonstrated to be an inferior projectile, dumping most of its energy advantage by 400 yards. But still, with the adjusted BC for the projectile I find that it still carries more energy to 600 yards than the MK262. After 600 yards the 115 SMK likely loses velocity more quickly as it enters the transonic velocity range and its ballistic coefficient will decrease, which is not accounted for in these plots, which assume BC doesn’t change.

OGW is a commonly used metric, and one must be careful to use it to compare ballistics between similarly powered cartridges, similar caliber projectiles, and bullet construction and purpose. All of the above rounds listed in the above chart are spritzer boat tail jacketed projectiles and so are reasonably comparable for performance against similar targets. In this case the OGW is a useful metric and like energy shows the clear superiority of the 6.8 SPC over the two .223 caliber cartridges against which it is compared. By 500 yards the 115 SMK has given up all advantage but the 110 BTHP continues to be the superior cartridge at least to 650 to 700 yards, the point where the round reaches the transonic region of flight where the ballistic coefficient will decrease.

Returning to NFA claim that the 6.8 SPC is limited to form factors of 1.15, we consider the 110 AB and 120 SST, and add them to the comparison above. I recently measured the muzzle velocity of MK262, Hornady 120 SST, and Nosler 110 Accubond ammunition, all from 18″ Bison Armory barrels. The following data adds the performance of the new rounds for comparison. The 120 SST starts out the heaviest and slowest of the bunch at 2540fps but with an i7 form factor of 1.077. The 110 Accubond was able to push 2600fps and has the best i7 form factor of the 6.8mm projectiles at 1.073.

In terms of drop, the .224 caliber projectiles cannot be matched, and the 110 BTHP is still the best contender in the 6.8mm group, though the 120 SST and 110 Accubond keep up nicely. The 115 SMK drops like a rock.

Velocity shows another story. Though the 75 BTHP is an outstanding projectile, the 120 SST nearly catches up to the MK262 and its 77 grain SMK by the beginning of the transonic region at 700 yards. The 110 Accubond starts out slower but effectively equals the 110 BTHP by 400 yards thanks to its low i7 form factor. The 115 SMK is left in the dust and again we see it is only suited for short range plinking and target use under 600 yards.

At short range, the energy of the 6.8mm SPC is in a different league than the 5.56 NATO offerings even with their modern relatively high power loadings. The 115 SMK really is a lousy bullet. The 120 SST is the superior cartridge/bullet combination, and if loaded as hot as the MK262 would outclass all comers by additional margin. The 110 Accubond and 110 BTHP could also be loaded in this way. We know from above that bolt thrust will increase, yet we have already mocked that concern and do not need to beat a dead horse.

Considering OGW is effective again because the 110 Accubond and 120 SST are designed for hunting and as such the comparison above is conservative and shows the 110 Accubond equaling the 110 BTHP, while the 120 SST outmatches all comers. Considering bullet design in the mix puts the 110 Accubond above the 110 BTHP.

My treatment is terse, and for non-writers it is challenging to produce a quality article for anyone interested to read. I’m guessing Nathaniel F thought the same thing and so we can forgive the shortcomings of his interesting article that provides many good points for discussion. I think I have demonstrated that the 6.8 SPC has some distinct advantages compared with the 5.56 NATO when loaded to modern specifications, much the way the 5.56 NATO improves with modern propellants and superior projectiles.