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.
On the firing line at 100 yards.
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.
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.
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’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:
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.
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.
Great fun was had by all. I wasn’t particularly good but doing better than my average. I earned the top novice award but then couldn’t accept it because I’m not an Oregon resident. I did get a CMP bronze pin though, my first acheivement in highpower. Highpower rifle is great fun and I recommend anyone interested to check out the various disciplines available at Douglas Ridge Rifle Club in Eagle Creek Oregon. I shoot F-Class and Service Rifle there and it’s a blast.
Oregon State Service Rifle Championship 2019, relay 3 getting ready to shoot sitting rapid fire at 200 yards.
For beginners, check out their Service Rifle Program, which is a sequence of friendly matches throughout the year geared towards novice competitive shooters. They have M1 Garand rifles and ammunition available and the cost is very inexpensive for a day of fun.
