MV x (12/twist rate in inches) x 60 = Bullet RPM
Quick Version: MV X 720/Twist Rate = RPM
As you can see velocity is a large player in the RPM game. Too many RPMs and the bullet jacket cant hold itself together and you get bullets that blow up in flight. But making the twist faster also has its equal part in the game. Its a combination of factors to consider.
Lets say that the hottest rounds I can find on brownells for your lightest given weight range, a 50 grain superperformance vmax, is flying 4,000 fps according to the brownells item description.
4,000x720/9=320,000 rpm Yeah, its getting up there but people do it every day.
Thats with the lightest bullet in your provided range at its fastest, so basically a worst case scenario as far as RPM goes.
At a more gentle 12 twist we get 4,000x720/12=240,000 rpm which is very comfortable.
So with that we come to our next consideration, how they will stabilize?
A 50 grain superperformance vmax is .772" long according to jbms length library, you get the following stability results via jbm and berger
9 twist
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Lets make it a 12 twist with that 50 grain vmax
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At 4,000 fps a 13 twist it goes unstable and at 10.6 twist it hits the 1.5 stability factor.
That is with the superperformance velocities, a 12 twist will work fine but wont be ideally stable.
If you find a slower loading with the same bullet it will of course be lower rpm and less likely to experience jacket failure but it would also be that much less stable. You would have to go all the way down to 2500 fps though before the 12 twist becomes unstable with a 50. So youre not standing on the edge or anything with a 50 grain and a 12 twist.
Going with a heavier bullet will only exasperate the stability issue in a slower twist barrel but the heavier bullet will also be going slower for less rpms and less likely hood of jacket failure.
Hornady 55vmax varmint express goes 3,275 by item description and is .811 long
9 Twist
3275x720/9=262,000 very comfortable
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12 Twist
3275x720/12=196,500
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12 Twist with a not overcharged superperformace loading under the 55 grain is on the edge of unstable, jbm says barely stable, berger barely not stable. A 60 gr would be worse. If you want to shoot the 50-60 grain then 9 twist is what you want.
If you wanted the lighter 30-50 grain bullets then a 12 would be more appropriate.
Looking at the 35 grain super performance thats going 4,450 fps.
4,450*720/9=356,000 and will almost certainly give you issues. In this situation the 12 twist you give you a much better 267,000 fps. But those little bullets only stay fast for a very short distance before petering out and then get blown all over with the wind.
