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Discussion Starter · #1 ·
I shot 60 rounds of S&B 110gr FMJ from my 18" Bison barrel this week and thought I'd share data, etc.
I built the rifle in December 2021 and this is only my second time shooting it (shame!)...rifle is described here:

Bison chamber described here:

Here is magnetospeed chrono data for 58/60 rounds fired (bayonet came loose in the beginning, operator error):
Font Magenta Electronic device Screenshot Number


S&B claims 2,556 fps for 16.1" barrel, so perhaps a little faster than expected.
I only had about 40 minutes to shoot 60 rounds (fast for me), so grouping was not really a priority...I mainly wanted to "make brass" for an upcoming deer load development (my first time for that). Shooting at 50 yards, groups (4-5 shots) were typically a little shy of 2 minutes; best group was .7 MOA.

There seem to have been a couple anomalies...some pressure signs (picture below) including swipes and cratered primers were on pretty much every case. And no, the primer did not pop out of the pocket, lol; I just placed it there for the photo.

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Also, about 4-5 cases did NOT fireform as much as the others...case base to shoulder datum measured noticeably smaller than the bulk...about 2-3.5 thousands smaller.
Using the Hornady "B350" comparator bushing, base to shoulder datum before firing was about 1.347"; after firing most cases were 1.3555-1.356. The handful of undersized cases measured 1.352-1.354. These "smaller" cases had distinct amount of fouling around case mouth, as seen below.

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Lastly, I think I'm very happy with the brass I'll be harvesting. The weight after decapping is nice and tight, probably 85+ percent falling between 116.5 and 117.5 gr.

Thanks for reading and I welcome any comments. I am a gun novice (but in my late 40's!) with just 6 years shooting, 4 years building rifles, and 2.5 years reloading.
I will be loading for my first ever deer hunting trip later this year, so I hope to post a lot about my load development, etc., and hopefully get pointers from all the experts for this fun and quirky 6.8 Special Step-Child of a cartridge.
 

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I kind of expect pressure signs on 6.8 or even 5.56 out of an AR so I don't worry. Running a heaver bolt carrier (Full Auto) and/or buffer, or getting an Adjustable Gas Block to reduce gas, can help slow down the system and help with swipes
 

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Those pressure signs are slight, nothing to be concerned with. In order for swipes to occur the case must be stuck to the chamber walls when the carrier moves to the rear and turns the bolt. You could polish the chamber a little or turn the gas down a little if that doesn't cause short stroking.
 

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Looks like you did, but make sure you remove the primers before taking the fireformed shoulder measurement. A proud fired primer can interfere with an accurate measurement.
 
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Discussion Starter · #5 ·
Looks like you did, but make sure you remove the primers before taking the fireformed shoulder measurement. A proud fired primer can interfere with an accurate measurement.
Thank you, yes that actually happened, at first. I compared my first few measurements to some previously fired cases and they seemed to be measuring long...removing the spent primers knocked it back down 1-2 thous.
Thanks to everyone commenting thus far.
 

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Those are not pressure signs in my opinion. Its just the way AR's function with the dwell time. Your bolt is turning to unlock and beginning to extract while the brass is still malleable. You dont have that issue in bolt action hence the pressure signs everyone mentions. In an AR the primer is the best indicator of pressure
 
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