I understand what the Twist Rate number is telling me, but can someone explain what I use this number for? What does it help me to determine?

Thanks,
Michael

You can over stabilize a bullet or under. Under can cause bullets to loose spin and keyhole. Or just bad accuracy.

Heavier bullets require faster spin, lighter bullets require less. But Specific gravity, type of material, velocity plays the role in the best stabilized twist.

A shooters rule of thumb is based off the greenhill formula.

The Greenhill Formula,
Twist=CD^2/L X SqrRoot(SG/10.9)


where:

• 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'sspecific gravity(10.9 for lead-core bullets, which cancels out the second half of the equation)

Let say you have a 300RUM, generally has 1 in 10 twist, optimizied for 180-200gr bullets. Where a 308 generally has 1 in 12 made more for a 150-165gr bullet.

taking note of this, it is much easier to under stablize a bullet then it is to over stabilize a bullet. some people actaully take adjantage of this, the belief being that under stabilized (but still accurate) bullets will tumble through game causing extra damage. examples of this can be seen in some millitary rifles using extra heavy bullets in slower rate of twist barrels and in some hunters using big 60 grain + bullets in .223 diameter centerfires with standard barrel when used against larger game. its also believed that bullets expansion is emphasize at high rpm's, produced by fast rate of twist barrels.

So as a person that doesn't reload, does this mean that certain barrels are optimized for certain weight bullets? Is there a rule of thumb for determining what off the shelf load would shoot best? As a hunter and not a target shooter, is this something I really need to be concerned with? I assume there is no danger in over or under stabilizing - correct?

With most factory guns, no concern. For example. you rarely see a 308win with a 1 in 10 barrel, and rarely see anyone shoot more than 180gr load. No big deal.

It could get harry when buying a target AR-15. Some come with 1 in 7 or even 1 in 6 barrel. Made for shooting 70-90gr .223 bullets. Where most factory ARs are 1 in 9 made to shoot 50-65gr bullets.

So generally speaking, the more twist per inch the heavier the bullet and visa-versa?

To a point, more to do with the length. But generally the heavier a bullet, the longer it is.