Look also at the relation bulletlenght, weight, max. speed and twist.
The shorter the bullet, the easier it strips trough the lands of the barrel which will ruin the surface of the bullet (and puts copper layers / strips inside the barrel)
A light (short) bullet can get more speed at maximum pressure than a long, heavy bullet. High speed and fast twist = stripping

So in a barrel with fast twist you can use light bullets at low velocity (reduced loads).
On the other hand, the maximum pressure will limit the speed required to stabilize a long, heavy bullet in a slow twisted barrel.

Dug out one of my old Hand-loaders digest books by John T Amber and I thought I would share this with you guys & gals.

Trouble from unpredictable and undependable bullets is the offspring of high velocity.

It was never heard of before the 30/06 appeared. Its Mom is speed in feet per second; its Dad, who gets little attention is rotational speed. Every time Mom travels 6 feet, Dad rolls over 7 times if the gun has a 10" twist.

Using the most exaggerated example I can think of, the 300whby magnum with 110gr bullet and 10" twist. Mom starts out at 3900 fps and Dad rotates 4500 times per second. The atmosphere belts Mom on the nose as soon as she gets out of the barrel and slows her down, but the resistance on Dad's rotation is much less. He arrives at the target rotating almost as fast as when he started.

We spin bullets to give them a bullheaded gyroscopical notion that they should not deviate from the path they started in. Unfortunately we can't do that without creating centrifugal energy as well, which pulls the outer part of any rotating object away from its center.

The centrifugal energy developed in a bullet rotating 4500 times per second is enormous. It would take the ordinary shop grinder 2½ minutes to make as many revolution as it does for the bullet to do in 1 second.

The 30'06 barrel was originally designed for a 220 grain bullet. Thet never changed the barrel when they changed the bullet and cartridge.
The 308 was designed for a shorter bullet 155 - 160 grain bullet. the slower twist is for the shorter bullet.

Actually the "weight" of the bullet is not what determines the twist but the bullet's length to diameter ratio.

So the equation is:



C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
D = bullet's diameter in inches
L = bullet's length in inches
SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

C = 150 for up to 2,800fps, 180 for above 2,800fps.

Dimitri

Dimitri wrote:

Actually the "weight" of the bullet is not what determines the twist.

Wouldn't it be more correct that bullet weight is a secondary factor compared for length and diameter? Example, bullets that are the same length and diameter, the lighter one (copper) will need slightly more twist than the heavier bullet (lead) to stablize.

The Specific Gravity of the bullet does play a role, however due to the square root function, the specific gravity is actually a smaller factor, then say the first part of that equation.

Which is why using 10.9 cancels out the 2nd part of the Green(something) formula and is generally not even mentioned.

Dimitri

If we do the math for a 308 bullet, in a lead jacket, that is 1.375 long, we get a twist rate of 1:10.35. However when substitution of copper (which would make the round weigh less) and using its Specific Gravity of 8.89 we end up with a twist rate of 1:9.35.

However cause the bullets that are all copper are generally longer, even a 1/4" extra length on the round makes the ammunition end up needing a twist rate of approximately 1:8. Which means out of a standard rifled barrel, all copper ammunition has a easier tendency to keyhole. Due to the much faster twist rates being required by the all copper ammunition.

Dimitri

Dimitri wrote:

much faster twist rates being required by the all copper ammunition.

Or run lighter bullets with their resultant faster velocities, faster trajectories, and more weight retention on impact.