Gentleman;
I believe the RPM given above are more than a bit conservative!
RPM of a Fired Bullet by Robert Treece
Volume 9, Issue 3
http://www.sierrabullets.com/index.cfm?section=techservice&page=xring&volume=9&issue=3#7Firearms have rifling twist in the barrel to impart stability to a projectile, keeping it pointed in the right direction without "wandering around", like a well thrown football.
RPM of a bullet is calculated from a very simple formula:
Velocity times 720 (a constant) divided by twist in the barrel.
Some examples to "chew" on: .22-250 Remington with a 1x12" twist can push 55 grain bullets to 3700 fps-figured thusly; 3700 times 720 divided by 12 equals 222,000 rpm.You could use 40 grainers high-velocity in a AR 20" with 1x7 twist and they can reach 3600 fps-that turns over 370,000 rpm; think what a 24 or 26" barrel would do with the added velocity.
Should you care? Yes, this spin also effects how a bullet expands and should be considered when selecting a bullet for a specific usage- remember that there are usually several choices in a given diameter.
If you don't know which one should be used, call!
SierraTechnical Support: 1-800-223-8799
Toll Free Support: 1-888-223-3006
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