.221 Fireball, heavy bullets, long barrels, and Varget

[Kernel]

221swede,

The simple Greenhill twist calculation is:

Twist Rate = 150 * (Bore Dia, in)^2 / (Bullet Length, in)

I use a modified version I call the Greenhill-Velocity twist calculation:

Twist Rate = ((0.0375 * Muzzle Velocity, fps) + 75) * (Bore Dia, in)^2 / (Bullet Length, in)

Basically, my equation modifies the constant, 150, accordingly to a linear equation dependent on velocity.

So, if we assume 2,400 fps and a 1.200" long bullet (how long is your bullet?), per simple Greenhill calculation the twist rate is 6.3. Per my Greenhill-Velocity equation the twist rate is 6.9.

The math says a 1:7 twist should stabilize your bullet based on velocity, and 1:6.5 based on the equation that does not use velocity. I think 1:7 would work fine. However, this is all theoretical. I would want to hear some real-world results from actual shooters. 1:6.5 might be better. The difference between 1:7 and 1:6.5 is pretty slight. But, I doubt a faster twist would hurt anything. When in doubt, most shooters (and gunsmiths) will opt for the faster twist.

 

[221swede]

hello
many thanks again
I am sorry for beeing so problematic..
but do you have any idea on how light/short bullets I can shoot with that brutal twist?
best regards
221swede

 

[flashhole]

That's a good question because it would leave one to believe you can over stabilize the bullet ... but to what detriment? I think the issue becomes one of bullet integrity. Will it hold together when spinning that fast? At the velocities you might achieve I think you would be fine with just about any quality bullet. Extremely thin jacketed bullets that are designed to vaporize on impact may not hold together in a fast twist barrel.