1:12 Twist 16" .308 Gyro STABILITY SCAR 20S Thoughts?

[yakmastermax]

Hey all,

I want to do a chopped 20S project, 15"-16" barrel length or so, in .308
Is the 1:12 twist okay for this barrel length and this caliber? I would like the gun to be optimized for 168-175 grain projectiles.

Thanks!

 

[yakmastermax]

I'm running some numbers through this Berger stability calculator: https://bergerbullets.com/twist-rate-calculator/
For 175 gr bullets with a 16.5" barrel generating MV of 2345fps or so, the 1:12 Twist is producing marginally stable SG values from 1.2-1.5 at sea level from 0 Deg F to 100 Deg F.
The same round at 2000 feet is producing SG factors that are marginally stable from 1.3-1.6 from 0 deg F to 100 deg F
At 4000 ft it is basically stable I think with SG factors from 1.4-1.7 for temps 0-100deg F.

For a 168 going 2450fps out of such a 16" barrel, at 0ft sea level, from temps 0-100deg F, I'm getting SG of 1.36-1.65
At 2000ft above sea level, from temps 0-100deg F, I'm getting SG of 1.46-1.78
4000ft above sea level, 0-100deg F, SG: 1.57-1.92


Bit more tabulated and easy to read:

175 gr, 16.5" barrel, MV of 2345fps or so:
0ft Elevation, 0-100deg F, SG: 1.2-1.5, marginal
2000ft Elev, 0-100deg F, SG: 1.3-1.6, marginal/stable
4000ft Elev, 0-100deg F, SG: 1.4-1.7, stable

168gr, 2450fps out of 16" barrel:
0ft sea level, 0-100deg F, SG: 1.36-1.65, marginal/stable
2000ft, 0-100deg F, SG: 1.46-1.78, stable
4000ft, 0-100deg F, SG: 1.57-1.92, stable

So overall it seems like perhaps if I don't go to 15" but rather keep it around 16-16.5", the 1:12 twist should work so long as I'm not shooting in the freezing cold at sea level, at least not with these 168-175 grain bullets?

 

[LRRPF52]

I personally have seen 16” SCAR-Hs in my DM courses that shot OK out to about 800yds, then it seemed like shot dispersion opened up noticeably for whatever reason. Could be the barrel heating up or gyro. This is at higher altitude too, 6300ft ASL, so that favors a marginally-stable bullet.

I’ve had guys in the same courses, same range, same conditions, shooting 20” SR-25s printing really tight groups on steel at 1000yds with 175gr, dimpled barrels, 1/11.25” twist.

I know if I was doing 16” .308, I would want 10 twist or tighter so any concerns about gyro stability are taken away up-front.

I have 2x .308s, both with 10 twist. I never shoot them though.