How to determine barrel twist with slower velocity?

[Sling]

I know that bullet makers specify barrel twist for each bullet but is there a formula for figuring this out? For example, the 50 BMG usually has a 15 twist barrel but when the same bullet is slowed down to 1200-1500fps, shouldn`t a faster twist be used? Also torque will be greater than normal if too fast of twist is used.

 

[GerryM]

You can find the answer to that at jbm ballistics.com
The program is free and easy to use. The Stability program

 

[mikecr]

JBM will work good enough if you use it well. There is also a Miller rule of thumb:

Gyroscopic stability(Sg) = 30*BW/((Tw/BD)^BD^3*(BL/BD)*(1+(BL/BD)^2)))*(MV/2800)^(1/3)/RO

BW=bullet weight, grains
Tw=twist, inches per turn
BD=bullet diameter, inches
BL=bullet length, inches
MV=muzzle velocity, ft/sec
RO= air density ratio from ICAO air density(ICAO=1.000)

Sg of 1 = tumble
Sg of 1.5 = fully stable

 

[gotcha]

Sling, You can also access a twist rate calculator @ Berger bullets. Their calculator will give you a good, better, best reading. I've found it to be adequate for my needs.

 

[Sling]

So should the stability factor be as close to 1.4 (above) as possible in the worse conditions that I shoot in? After doing some research, if the stability factor is around 1.7, then I should be good. I know that even a 3 is good but then torque becomes a factor right?

 

[mikecr]

You got it Sling.
It's better to be stable(+), than stable(-).

 

[Sling]

What about solid copper or all brass bullets? Does the same apply?

 

[mikecr]

It's the same with solids.

 

[Sling]

Thanks Mike. I have been calculating alot of bullets since yesterday and it makes a person wonder how a factory rifle shoots as good as they do.