Incoming: Understanding Rifle Twist

Bullet-Rifle-Barrel

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.

What do you think about rifle twist rates, bullet stability and long-range shooting? Sign in and leave a comment below.

This letter originally appeared in the June 12, 2014 issue of Gun Digest the Magazine.


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2 thoughts on “Incoming: Understanding Rifle Twist

  1. Ranchman

    Good article, it sheds light on the eternal mystery (to me, anyway) of correct rifling twist. I’m surely no expert but, I know that bullet weight is important to correct twist. My AR in 5.56×45, with a 1:9 twist, likes bullets no heavier than 62 gr. With a 1:9. anything heavier will yaw in flight. A 1:7 twist will accurately shoot the heavier, longer range bullets but, the lighter bullets–55 gr. and less–don’t like the 1:7, so maybe a 1:8 would be a happy medium? I don’t know. My bolt action model 10 FCP, in 7.62×51, likes the rifling mentioned in the article, 1:10, which leads me to believe the firearm mentioned is also a .308. I’m just now getting into longer range shooting, out to 1000 yards. Although my particular firearm likes the 168 gr. BTHP (especially the Sierra MatchKing), which is typical for this caliber, the heavier bullets up to 185 gr., do much better at long distance, even with the same 1:10 twist. Anyway, thanks for the info. Rifling twist is something I think we will be discussing for quite some time.

  2. dragonwood_94

    I dabble a little with ballistics and I must agree with TC Knight. Though I agree there are a few notes others might consider. Long range bullets need a good ballistic coefficient (BC…makes the trajectory flatter) hence they become longer and heavier than most average bullets of any specific caliber used for other than long ranges shooting. The length and mass work together in unison with the twist rate to stabilize the bullet. As well as the BC, each barrel made has a particular taste in bullets. Some bullets shoot better in certain guns. For example I have 2 different custom actions in 300 Weatherby Mag, one a shorter heavy barrel, the other a couple of inches longer but much lighter barrel. Both are match grade and have the same twist. The short barrel loves 150gr and I get 1/4 MOA while the other enjoys shooting 180gr and heavier. A rifle is a total package when matched with the round that shoots best through it. There are many variables for accurate long range shooting, most of those variables we cannot control, including the person holding the rifle. We can get close tolerances on many of the variables, but in the end the question is always what is the target at the terminal end of my round?

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