Thread: 500 Yard Rifle
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Old May 22, 2013, 01:46 PM   #56
Bart B.
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Join Date: February 15, 2009
Posts: 4,986
I think Metal god's referring to as yaw is actually the coning motion of the bullet's nose about the trajectory axis. As the bullet's spin axis is always parallel to the trajectory, its nose will always be a tiny bit above the trajectory; so will the center of its tail. That happens when the bullets are not perfectly balanced about their form axis. As 99.9% of all bullets are not perfectly balanced, each one will have a bit of coning motion, or nutation as it's sometimes called, about the trajectory axis. The more nutation a bullet has, the more drag it has. More rpm's caused by fast rifling twists causes more centrifugal forces that cause that nutation. Which means the BC will be less than those perfectly balanced ones. The center of that coning, or nutation, will be above the trajectory axis.

Most of the best bullets will have up to a 1% spread in BC at most, but it's still enough to open up groups non-linearly as range increases. When this adds to the velocity spread's effecting bullet drop, it's no wonder long range groups do not subtend the same MOA value the ammo had at short ranges. Those well balanced bullets leaving at the lowest velocity can strike the same point on target as a bullet a bit unbalanced leaving at the highest velocity. The opposite happens when things are reversed; bigger groups.

There is some evidence that when bullets are spun way, way too fast, they tend to stay more paralled with the bore axis and do not nose over like passed footballs do enroute to the receiver. At this extremely high rpm rate, the normal unbalance of bullets will then cause disasterous effects on accuracy.

Sierra Bullets' has had excellent info on this stuff in their reloading manuals. Even time of flight measurements showing how different twist rates for the same load change the bullet's BC.

Short range benchresters shoot their bullet with the slowest twist possible for best accuracy. They'll adjust the powder charge a tenth or so for temperature changes; cold air's thicker than hot air and bullets need to be spun a bit faster to stabilize them so they add a tenth (or one click on their powder measure). 120-gr. bullets in the .30BR shoot great from 1:17 and 1:18 twist barrels.

A 1:10 twist from a 24" long .308 Win. barrel's a bit much for any bullet lighter than 200 grains, in my opinion, for best accuracy. In sub zero weather, however, it may be about perfect.

csmsss says that's pitch, or vertical deviation from the intended path. Do you mean the bullet takes a higher or lower arc to the target than normal?
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Last edited by Bart B.; May 22, 2013 at 03:36 PM.
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