Cavity Back Bullets has a new 175 grain bullet (lead free but not cavity back). It’s made to expand reliably down to 600 fps to 2.5x caliber. I’m going to get them out to the range for a test in the next week or two, looking forward to seeing how they perform. It will be nice to have an alternative!
Left to right: 160 Winchester (6.8 Western), 175 CBB, 200 Woodleigh, 120 MKZ
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 |
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.
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.
More details coming soon!
UPDATE!
Get your Case Volumizer at Bison Armory now!
Bison Armory Case Volumizer – Bison Armory Store
View the instruction videos on YouTube:
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
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.
A customer sent me this photo of his fine Bison Armory based rifle. The 22″ 6.8 can reach out there. The customer got the last of our Mega billet side charge upper receivers. Mega is gone and there won’t be any more of these fine upper receivers. I built all my long-range AR-15 rifles with these SBU side charge receivers, they are bulky and burly and absolutely perfect for the job.
I had some time yesterday evening to hit the 1000 yard line at Douglas Ridge. My intention was to shoot the 80 SMK, but somehow I packed the wrong ammo box which only had 100 rounds of .223 Rem loaded with 77 SMK bullets for an upcoming service rifle match. Ugh!
Fortunately I did have some 88 ELD factory ammo and 88 ELD hand loads. The factory stuff is definitely tamer than my hand loads. The brass ejects nicely at about 3:30 with a silencer and my 24″ 1:6.5 twist barrel. Come up at 1000 yards with the factory ammo is about 10 mils, while my hand loads were centered at about 9.5 mils. Wind was only up to maybe 5 mph but was switching from up range to down range and from left and right. Definitely a tricky situation to put good groups together.
On another note, the 1000 yard targets from Midway were perfect for my Caldwell target camera system. These photos are from up close with my phone. The first one is my sight in target that I had the Caldwell camera pointed at. I used this one to chase my zero for the different loads I was shooting:
That little bug was living dangerously. Once dialed in on this target I adjusted what should have been for zero then shot blind on the other targets. First the factory Hornady 88 ELD:
A little high, a little right. The wind had changed from up rang (from target to shooter) to left to right, and so bullet impact was right and high. There aren’t any flags up and there was effectively zero mirage today so I didn’t bother doping the wind. I shot these pretty quickly too, about 5 seconds between shots. Next some hand loads with RL17:
This was after adjusting 5 clicks down (0.1 mil/click). Again I’m chalking the high shots to changing wind conditions. I wasn’t letting the barrel and silencer cool either, which probably didn’t help with my accuracy, again shooting this string in under 30 seconds.
Conclusion: .224 Valkyrie can reach out there easily. I didn’t miss paper at 1000 yards with any rounds. It helps to have 42″ square targets, which you can get at Midway USA.
I took the .224 Valkyrie out to the 1000 yard line on Thursday at Douglas Ridge Rifle Club in Oregon. It’s about time I got to stretch the Valk’s legs. I was shooting the 95 SMK in a Bison 24″ 1:6.5″ twist barrel. My muzzle device was an Elite Iron brake that I normally use with their Bravo 1 silencer but my DOPE for my previous sight in was taken without the silencer so I kept it like that for this session. Here’s the view from the targets looking back at the firing position:
The target positions are behind me. The targets in front are for 200 to 600 yard service rifle competitions. Here’s the view from the 900 yard firing line:
Turns out it helps to get on paper to know which firing line you are at. I thought I was at the 800 yard line and I ended up wasting quite a bit of ammo getting on paper. Checking against my DOPE and ballistics data I was quite puzzled at the almost 7.8 mils of come up needed to get on paper when I had computed something more like 6.8 mils were needed. Thanks to the low recoil of the .224 Valkyrie I was able to see the rounds hitting the dirt which told me I was a ways off. Wasting ammo with my bald spot blasted by the late afternoon sun wasn’t my idea of fun. But I did get on paper and then back to the 1000 yards line we go. Shade is a good place to shoot from.
The .260 Rem got to try 1000 yards too. The DOPE for that one was way off as well, you’d think that would have told me something. I got on paper and then changed targets. I should have taken a photo of the Caldwell Target Camera LR system that I was using. This thing was the best $350 I’ve spent in a while.
I have no Android or iOS devices So I had to improvise. You can log into the camera through a website. The IP address of the camera is marked on the side, which in my case is 192.168.0.3, and then provide the username admin with the password 12345 and you get to a menu written in Chinese. Selecting the second item in the menu gets you to the live feed from the camera.
A note about this system: I couldn’t see a single hole in the target black though I hit it several times. Hits in the white are clearly visible. I recommend using these targets for 1000 yard shooting with the Caldwell Target Vision camera system:
I haven’t used this target yet but I think it will work well. It’s 42″ square and I think light enough to see the bullet strikes. I was using white cardboard which worked really well too, but didn’t cover the target black behind it completely so it I could not make out the bullet holes that were not in the white. I’ll report back after my next session later this month about how the IBS targets worked out.
As one might guess, getting on paper at 1000 yards given my confusion about which line I was shooting from at 900 yards, a little work was needed to get on paper at 1000 yards. Again puzzling because once on paper my come up was 9.2 mils when I had expected 10 mils from my ballistics computations and then probably more given the 7.8 mils I thought I needed at what I thought was 800 yards. Both the .260 Rem and the .224 Valkyrie were consistently inside 12″, which is pretty good for me given that I’d never shot at this distance before. Winds were consistently inconsistent but at about 5 mph. The direction changed often and sometimes died out completely only to come back again 30 seconds later. First time out at 1000 yards, the range all to myself, what’s not to like:
Holy mackerel, my nose isn’t nearly that big in real life, I swear! Now that I’m dialed in, I’ll be shooting some groups my next outing instead of just spraying all over the target while constantly adjusting my windage and elevation. Conclusion: .224 Valkyrie can shoot at 1000 yards. Next time: 80 SMK loaded to about 2850 fps in the 24″ barrel. Oh yeah, after wrapping up for the day I noticed that the 800 and 1000 yard berms were really close together. That’s when the dam broke and I realized I’d been shooting at 900 and not 800, argh!
We’re bringing Bison Armory quality and value to the .224 Valkyrie world with 20″ SPR and 22″ Fulcrum rifle barrels! Barrels are available for pre-order now and should start shipping in March 2018. Upper assemblies will be available when the barrels are in stock. Check here for barrels and here for upper assemblies.
Bison Armory .224 Valkyrie barrels will be available with Stag, JP, and LWRC bolts, and feature the following specs:
1:7 twist button rifling
416R stainless steel
5/8-24 muzzle threads for maximum accuracy
Whitworth Tool (SPR) and BAT Machine (Fulcrum) barrel extensions with M4 feed ramps
http://bisonarmory.com/scratch-and-dent/
Super prices on some barrels and upper assemblies that we need clear out.
Bison Armory is happy to announce that we’re finally making 308 Win upper assemblies! We’re using Aero Precision M5 upper receivers that are 100% compatible with DPMS LR-308 lower receivers. Accuracy is outstanding, and you can pair your 16″, 18″, and 20″ .308 Bison Armory Fulcrum barrel with a standard .308 Bolt and carrier or upgrade to JP Enhanced Bolts and LMOS carriers. Forearms from SLR Rifleworks and Aero Precision complete the upper assemblies.
The complete upper shown weighs in at 6.8 pounds. Here’s my last 10 shot group using my personal recreation of FGMM 168 grain loads
You can check out our new upper assemblies here! 18″ upper assemblies are available now, and 16″ and 20″ will be available soon.