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Old February 27, 2013, 01:09 PM   #20
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,985
Rifling twists for the .308 Win barrel. . . . . .
1:12 twists in 24" barrels have given match winning and record setting accuracy at ranges up to 1000 yards with bullets from 172 to 190 grains. 1:11 twist barrels were "the one to use" for 168's with reduced loads up to 190's with max loads when the .308 was the only thing used in most high power competitions. And the rate bullet rpm's slows down is about 10% the rate their velocity does; they don't always spin 180,000 rpm's (3000 fps from a 1:12 twist) all the way to 1000 yards.

Velocity changes per inch of barrel length. . . . . .
Too many tests by folks and SAAMI have shown that the .308 Win. gets about 25 inches per inch of barrel length from 20 to 30 inches. Shorter, they're slower; longer, they're faster. That's using the same barrel starting out long then cutting off an inch for each series of test shots.

Best spin rate for bullets. . . . .
Whatever combination of muzzle velocity and twist that spins them at the right rpm to stabilize them all the way to the target is right. A 180-gr. bullet from a .300 Win. Mag's 1:13 twist barrel will shoot very accurate. That same bullet from a .30-06 or .308 Win. will do well in a 1:12 twist. From a .300 Savage, it'll need a 1:11 or 1:10 twist. Spinning 'em just fast enough to stabilize them is what short range benchresters do. Spinning them in the middle of their best rpm rate is best for longer ranges. Too fast a twist will cause those more unbalanced ones to shoot less accurate; they'll jump too far off the bore axis when they leave it due to cetrifugal forces.
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