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Incoming: Understanding Rifle Twist
Editor's Note: One of our readers recently took us to task over an article that covered rifle twist rate and bullet stability in relation to long-range shooting. His response is thought-provoking.
I read with interest Dick Jones’ article “Shooting’s Ultimate Challenge” (March 27). Although Jones is no doubt an excellent long range shooter, and his qualifications are good, I must point something out regarding the side bar “Understanding Twist.”
Jones wrote that “…as the bullet slows down, the spinning motion that stabilizes the bullet slows, and it can become so slow the bullet becomes unstable.”
This statement is rather misleading. Although the spinning of the bullet most certainly slows down over time, its rate of deceleration during the flight time of the bullet is, at worst, negligible. For many of us, our first experiments in physics involved the rotation of toy tops and the forward travel of marbles. If you have ever spun a top, you know that there is little resistance and the spin speed takes a long time to slow down, maybe minutes.
At the 3,600 rotations per second of Mr. Jones’ bullet when it leaves his 1:10 barrel, it would take many minutes for the bullet to slow down enough to affect its stability, but it takes less than 2 seconds for it to reach it’s 1,000-yard target. The sidebar suggests that 1:10 twist is the ultimate rate for this caliber with this bullet, and 1:12 twist rates will not do. That, however, may not necessarily always be the case. Berger Bullets ballisticians recommend the slowest twist to correctly stabilize its 185-grain VLD match bullets as 1:12. This is the recommendation for a bullet actually designed for long-range competition.
The rate of spin for a certain bullet has everything to do with “calming” the bullet down shortly after it leaves the barrel, and less to do with maintaining its stability at long ranges. The current trend in ever increasing rates of twist is introducing a whole new set of problems to the long-range shooter. Gun writer Jim Carmichael once wrote that “No shooting subject is more likely to make one sound like an expert, and at the same time prove him a fool, than a discussion of rifling twist.” I hope I haven’t proven myself a fool but a contrary opinion is always helpful to initiate thought.
—T. C. Knight, Davis Creek, Calif.
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This letter originally appeared in the June 12, 2014 issue of Gun Digest the Magazine.
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