Twist rate calculators and stability predictions

[theloyalsavage]

I own one of the early model 16 270 WSMs with the 1 in 11 twist barrels. I have shot 150 BST’s, 140 AB’s, 130 BST’s, and the Barnes 129 LRX a long bullet.

all the stability calculators, including Berger’s, predict that all the bullets would be barely marginal stable, right on the threshold of instability. Even the 130 is predicted to be marginally stable.

all of these bullets shoot great from 100 to 500 yds. The longer bullets shoot the best. The LRX is king.
never seen anything that looks like a keyhole

So

Dont let someone tell you you need a rifle with a fast twist to shoot long bullets. If your rifle shoots well chances are it’ll shoot long bullets too. (Within limits)

Gonna test even longer bullets soon. Some bergers. Wanna find the limits for this 11 twist

all this 1 in 7 1 in 8 stuff is just getting irritating, especially for hunting rifles that shoot 90% of shots within 400 yards

[charlie b]

Others of us have pushed the limits as well. 77gn SMK's in a 1:9 twist barrel. I have a 1:8 twist 6BR that stabilizes 105gn Bergers but not 108gn Hornady's.

The formulae are not that accurate near the limits. Many factors but bullet shape is one of them. The analysis assumes the bullet is a football shape with an even density throughout. Designers like Berger alter the interior of the bullet to change where the Center of Gravity is in relation to the Center of Pressure, which changes the stability of the bullet. That doesn't even account for the length and angle of the boat tail which dictates where the shock waves are on the bullet, which also affects the Center of Pressure location.

But, need to pay attention to some other things as well. Temperature affects air density a lot so colder air can cause a slightly stable bullet to become slightly less stable. If you test on 70 or 80F days and hunt in below freezing you could run into that problem.

Muzzle velocity plays a role as well since it changes the rotational speed of the bullet. Lower velocity means you need a bit higher rate of twist. You can play with the formula to see the effects of different variables.

Basically it means if you want to push the limits, do so and carefully note the conditions for future reference. It may work and it may not, but, the only way to tell is to try.