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.
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.
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.
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.
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.
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.
The Alpha version of Dr. Triplett’s Cartridge Case Volumizer (DTCCV) is coming soon! Measure case capacity in grains of H2O in around 2 seconds per case using your reloading press and Windows 10 computer.
The Alpha version is intended for scientifically minded early adopters who are reasonably tech savvy.
I finally dialed in the 88 ELD with my latest hand loads. Until now the factory Hornady 88 ELD has out-shot what I was doing in the reloading room. I noticed that the factory loads are very light, to the point that the action cycles very mildly. I backed my loads down a bit and loaded them closer to the factory 2.260″ OAL, with my loads coming in now at 2.270″.
The secret sauce is 26.5 grains of Winchester 760 powder, Starline brass, and Winchester small rifle primers. I didn’t measure muzzle velocity but the loads still seamed hotter than factory, so I’m guessing 2650 fps from the 22″ barrel (factory shot 2650 fps from my 24″ barrel.)
I was able to shoot two 1 MOA groups, one with 9 shots and the other with 7, at 100 yards. I need to fine-tune this load now and then stretch its legs, but I expect good things given how the factory loads shot at 1000 yards last year from the 24″ barrel.
The group below is pretty satisfying at this point. Notice how you could combine those shots in different ways to get several sub-moa 5-shot groups. Never trust low round counts. I plan to refine this load a bit more, probably just drop to 26.4 grains and see how it shoots. I’ll put up a 20+ round group to see what we get.
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.
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
Simulated group 2
Simulated group 3
Simulated group 4
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.
