Barrel Twist Rates

[Bart B.]

Barrel twist rates for conventional rifling recommended by Shilen Barrels in this link:

http://www.shilen.com/calibersAndTwists.html

30 Caliber:
7" for heavy VLD bullets and/or subsonic ammo.
8" for bullets heavier than 220 gr.
10" for bullets up to 220 gr.
12" for bullets up to 170 gr.
14" for bullets up to 168gr.
15" for bullets up to 150 gr.
17" for bullets up to 125 gr.

22 Caliber:
7" for bullets up to 90 gr.
8" for bullets 80 gr. and under
9" for bullets up to 70 gr.
10" for bullets up to 65 gr.
12" for bullets up to 63 gr.
14" for bullets up to 55 gr.

Muzzle velocity must not matter.

[Unclenick]

MV matters, but not immensely. Extra velocity gets you ahead on bullet stability factor by driving the bullet fast enough to get to a lower drag coefficient. That coefficient peaks at Mach 1, where the "sound barrier" makes it greatest and then trails off with added velocity at rates that depends on the bullet shape. If the drag coefficient were constant, velocity would have no effect at all.

The reason for the above is that if the drag coefficient were constant, drag, including the overturning drag that wants to make your bullet tumble, would go up simply as the square of velocity. The ability of spin to stabilize a bullet against overturning drag goes up as the square of the rate of spin. So when you doubled velocity you would quadruple drag and because going through the rifling at twice the velocity doubles the rate of spin, you would also be quadrupling the ability of the resulting spin to stabilize the bullet. The two effects would cancel out. But because real drag is higher the closer you get to Mach 1, you end up picking the twist to be adequate at your lowest muzzle velocity and then when the velocity is increased, the drop in drag coefficient means you have some extra spin.