Comments on: Calculating Bullet RPM — Spin Rates and Stability

By: Josh

Josh — Sun, 26 Jan 2020 18:42:29 +0000

JD – sorry,but your math really is flawed, by confusing inches with feet. “There are 428.57 7inch segments in 3000 feet” is wrong. There are 428.57 7 FOOT segments in 3000 feet, so your calculations are off by a factor of 12. Oops.

By: JD

JD — Wed, 22 Jan 2020 23:26:34 +0000

I came up with a wholly different way to calculate spin which actually matches slow motion video.
I have a 1:7 twist rate 5.56. The bullet rotates once in 7 inches. MV is 3000fps. There are 428.57 7inch segments in 3000 feet meaning the bullet will rotate 428.57 in one second.

EDITOR: NO. 3000 feet = 36000 inches. Which, divided by 7, equals 5142.85. Accordingly, all your other calculations are wrong.

Multiply that by 60 and the rpm is 25,714 RPM. A rotational rate over 200,000 would mean the bullet is spinning faster than almost anything on earth and on slow motion video would spin at what looks like full speed even though it’s slowed down 25,000 times. Or better using your calculations the bullet would rotate more than 3300 times per second. My 428 per second is much more rational than 3300 times per second.
The calculation should be MV/twist rate X 60. Or Muzzle Velocity divided by twist rate inches = number of times bullet will rotate In 1 second. Multiply by 60 for rpm.
For a Glock 17 Twist is 1:9.84. MV 1200/9.84=121.95 X 60 = 7317rpm. That makes much more sense than the number I get with your formula.
I’m not a math whiz so feel free to tell me why I got this wrong and why slow motion video of bullets doesn’t show astronomical rpm rates your formula suggests.

By: Dr KK Chand

Dr KK Chand — Sun, 24 Feb 2019 16:20:44 +0000

Dear Sir, Excellent Article. Regards. KK Chand

By: Steve parsons

Steve parsons — Fri, 18 Mar 2016 17:45:58 +0000

The rotation of the bullet is simply making the bullet lighter, perhaps even providing some sort of anti “gravity” effect. The strange properties of masses spinning at extremely high rpms has been documented for a long time now. A demonstration of the strange weight reducing principles of high rpm spinning objects can be viewed herehttps://m.youtube.com/watch?v=GeyDf4ooPdo

By: Kyle Newman

Kyle Newman — Sat, 28 Nov 2015 16:42:18 +0000

I’m glad you stated that the destabilizing affects of the air increases as velocity increases, but you also are saying that you can make a bullet that would normally be unstable at lower velocity more stable when pushed at higher velocity. Now I understand that faster means higher rpms which implies more stability, but is it really that simple? I can’t help but feel like your calculators and equations aren’t taking into consideration the increase in instability forces when velocity through the air increases.

By: Jeff

Jeff — Sun, 25 Oct 2015 12:44:52 +0000

To a laymen like me. What would be the optimal barrel twist for a hunter using .308 Winchester full jacket military surplus ammo? Thanks any help would be appreciated as math and me were not friends in school.

By: B.C.

B.C. — Fri, 10 Oct 2014 19:36:49 +0000

Get Smart – So I’ll call mine the Router Effect – Late ’80s A hunting buddy and I experimented with a Ruger Mini-14 to see if it would work for Deer Hunting. To 1st sight in, we used a Mil-issued M193 round shooting into a stack of abandoned wood that was “Pithy” inside the bark. We split the log in 1/2 to find the FMJ bullet had kinked almost 90* and cut a channel into the 18″ log, bullet still intact. It had rotated,horizontally, not tumbled end for end, and veered to the right – Leaving a channel of chewed up wood, the entire path.
I’ll call it the “Router Effect” if it makes you feel better.

By: Get Smart

Get Smart — Mon, 15 Sep 2014 07:47:37 +0000

Buzz saw? Foreword?
For Pete’s sake people! You all must realize that we have high speed cameras, as well as clear ballistic gelatin these days. Do you not?
How is any of this “buzz saw” garbage even a conversation?

By: Starfire

Starfire — Tue, 03 Sep 2013 22:26:12 +0000

I didn’t notice any difference when shooting slugs from a smooth bore shotgun vs a rifled shotgun, so I think the buzz saw effect is rather minimal. Can’t wait to try and find a difference now that I am looking for one.

By: John

John — Wed, 14 Aug 2013 23:07:48 +0000

The effects of bullet spin are obviously important, but the discussion was about calculating bullet spin. It might seem like nit picking to argue about the formula since it gives a consistant indication which satisfies the needs of shooters whether it is accurate or not.

My objection is that in theory nothing can accelerate to say 2800 fps in zero time. If a bullet is at rest and reaches a muzzle velocity of 2800 fps, the average velocity within the barrel is only 1400 fps if the acceleration is linear. Therefore,the spin rates obtained using the given formula would be twice the actual spin rates. But, acceleration might not be linear and might depend on the characteristics of the propellant used.

Bullet and firearm manufacturers must have a better handle on this than I do, before I found this discussion i tried to derive my own formula and didn’t know if using the muzzle velocity was valid.

The SG calculator removed some of the doubts I had about muzzleloader rounds, but with the wide variety of bullets in use I wonder if the 1:28 twist is suitable for all of them.