1:75 vs 1:8 - anybody tell the difference and if so , where?

[lazyengineer]

My opinion on this issue has evolved.

I set twist rates for the worst-case scenario, which I define as a 12" Grendel on the Alaskan coast in winter. So I go to the Berger Twist Rate Stability Calculator and select from the drop-down menu a given 6.5mm bullet, then enter 2000 fps for muzzle velocity and 0° F temperature and 0 feet altitude. Then I start playing with twist rates.

In prior years, my assumption was that I should specify barrels for that rare shooter who might want to use a 140-grain bullet in the Grendel so I specified a 1:7.5 twist. Now, however, I don't think a 140gr is realistic in the Grendel, and that 130gr is the maximum realistic weight. So using Berger's calculator with my worst-case scenario and selecting their 130gr Hybrid OTM, I find a 1:8 twist gives full stability to my SBR. (Not that I'd really be worried about losing 1% or 2% ballistic coefficient from an SBR!)

I used to think there was no downside to over-stabilizing a bullet (talking 6.5 Grendel here, not about pills coming apart at 4,000 fps), because a Brian Litz study said it could help accuracy at long range and the fps loss from tighter twist rates was ridiculously low — about 1.25 fps per twist inch.

However, there is also the consideration that microscopic imperfections in bullet manufacturing — the microscopic voids in lead cores or the non-concentricity of both jacketed and monolithic turned bullets — can cause a bullet to slowly wobble off target due to uneven centrifugal forces. These imperfections are exacerbated by higher twist rates.

So, on the one hand we want tighter twist rates for accuracy. On the other hand, we want looser twist rates also for accuracy. What to do?

Set the Goldilocks "just-right" twist rate. For mass-produced 6.5 Grendel barrels shooting a max 130gr bullet weight, my opinion based on the reasoning above is that it's 1:8.

The last part in particular. Personally I run 1 / 9 in 20", and run 1/8 in everything shorter - and have been happy. I have little desire to run 1/ 7.5, but I suppose if you are planning on running 140's, in a 12", it would matter.