Twist Questions

[joesr]

I've seen alot of threads about what twist barrels will work with heavy bullets for 6mm's.

First, if you are shooting a button rifled barrel the actual twist of your rifling is probably + or - 1/2 inch of what you ordered. You need to keep this in mind.

This is why some barrels work and some don't.

Cut Rifled barrels are all but exactly what they claim to be.

The difference is in the manufacturing process.

A really easy way to determine if you can possibly shoot a 105 gr class bullet is to multiply your twist rate by 350. This will give you an idea of the required velocity to rotate the bullet fast enough to stabilize it. An 8 twist barrel requires 2800 fps and a 10 twist requires 3500 fps.

Bullet manufactures will not tell you to do this because twists faster than 8:1 require exceptional velocities and bullets have a difficult time staying together and performing at these velocities.

I hope this helps,

Joe H

 

[scout1]

joesr, do you have a list of these multipliers for different weights and calibers of bullets.

Thanks, Scout1

 

[Obtunded]

I have long felt that bullet manufacturers should publish optimal RPMs rather than twist rates for their bullets. There are many examples of where bullets are used in cartridges that would be considered quite atypical,ie; The 'Whisper' series of cartridges). Generating RPM would be dependent on velocity times twist rate, but knowing your target rate of spin would be much easier if that sort of ballistic info were provided.

There have been many discussions regarding the use of heavy 6's with PPC cartridges, but velocities would be insufficient to generate proper bullet RPM's in a standard 1:8 twist.

 

[RGRobinett]

'A really easy way to determine if you can possibly shoot a 105 gr class bullet is to multiply your twist rate by 350. This will give you an idea of the required velocity to rotate the bullet fast enough to stabilize it. An 8 twist barrel requires 2800 fps and a 10 twist requires 3500 fps. ' JoeH

This is an incorrect over-simplification. I'd suggest looking-up the JBM ballistics DRAG/TWIST page and entering the measurement DATA from the particular bullet. In short, if an 8' twist produces marginal Sg at 2800 FPS, you won't find much improvement at even 4500 FPS. For a given bullet, as velocity increases, maintaining Sg dictates higher RPM. Here's an example: a particular bullet,in this case, one of marginal Sg), at 2500 FPS, via the particular twist, produces 90,000 RPM and Sg of 1.206. Same bullet and barrel, but velocity increased to 4000 FPS, produces 144,000 RPM; Sg = 1.266 still FAR short of the desired 1.4 Sg! All of that velocity only affects a change in the second,meaningless)decimal place. There's the rub: with any cartridge, loaded to anywhere near 'normal' pressure, leave velocity out of the puzzle. Proper Sg,1.4 or greater)is best achieved by matching the bullet to the twist or, vice-versa: this does not always mean reducing/increasing bullet weight - there are many ways to skin that cat. RG

'Bullet manufactures will not tell you to do this because twists faster than 8:1 require exceptional velocities and bullets have a difficult time staying together and performing at these velocities.' Joe H

A 1:10' twist is slower than a 1:8.1' twist; not faster.

P.S. Here's the link to JBM: http://www.eskimo.com/~jbm/cgi-bin/jbmdrag-5.0.cgi