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Range Report RPM Of A Bullet In Flight

Muttt

Muttt

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Minuteman
Jun 3, 2010
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I was having a discussion with a friend the other day and we were talking about how fast a bullet spins in flight. He was of the impression that it actaully spins slowly. But, I'm of the impression that the RPM of the bullet would be up into the tens of thousands of revolutions. Anyone with an Ivy Leauge math brain have any idea??? Just curious. Not that it would make any difference with either of our shooting.
 
Re: RPM Of A Bullet In Flight

Umm, how about hundreds of thousands?

Assume that the bullet at the muzzle is going 2600 feet per second in a barrel with a twist that is 1 turn per foot (1 turn in 12 inches.) The math, then, is pretty simple. 2600 revolutions per second X 60 seconds per minute = 156,000 RPM.

Edited to add: By the way, the math faculty of most Ivy League universities sucks - with the obvious exception of MIT, which isn't really Ivy League anyway.
 
Re: RPM Of A Bullet In Flight

Take a typical 308 Win with a 1/12" twist.

2600 fps MV would mean 2600 revolutions per second (or 156,000 RPM) as it leaves the muzzle.
 
Re: RPM Of A Bullet In Flight

Another way of thinking about twist is that for a 1/12 twist barrel the bullet will turn once every foot of travel or 300 revolutions in 100 yards or 900 complete revolutions in 300 yards. Not a lot of turns but it's doing it in a short time. A 1/9 twist barrel would spin the bullet 400 times in 100 yards.
 
Re: RPM Of A Bullet In Flight

From the Speer No. 13 reloading manual regarding the .223 Rem (5.56mm NATO) "When fired at 3200 fps from a 1:7 twist rifle, the bullet is rotating at over 300,000 rpm when it leaves the muzzle". I believe they are referring to "light" varmint type bullets in the fast twist barrels. I've never done the math, but just think of even a pistol bullet, say a 230 gr. Golden Sabre, fully expanded (sharp jacket petals and all)passing through you gut spinning like a meat auger.
 
Re: RPM Of A Bullet In Flight

It does not work that way... the 45 ACP has a twist of 1-16", so the penetrating bullet only rotates about 3/4 of a turn in the typical 12" or so of penetration.
 
Re: RPM Of A Bullet In Flight

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rg1</div><div class="ubbcode-body"> Another way of thinking about twist is that for a 1/12 twist barrel the bullet will turn once every foot of travel or 300 revolutions in 100 yards or 900 complete revolutions in 300 yards. Not a lot of turns but it's doing it in a short time. A 1/9 twist barrel would spin the bullet 400 times in 100 yards. </div></div>

This....^

The bullet from a 1:12 twist turns 3 complete revolutions every yard it travels down range....and is slowing down the instant it clears the crown, although the rotation is slowing at a MUCH slower rate than the forward progess.

It's not a freaking tazmanian buzz saw coming out of the muzzle........
 
Re: RPM Of A Bullet In Flight

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TiroFijo</div><div class="ubbcode-body">It does not work that way... the 45 ACP has a twist of 1-16", so the penetrating bullet only rotates about 3/4 of a turn in the typical 12" or so of penetration. </div></div>

See, I should have done the math. Sure sounds nasty though don't it?
wink.gif
 
Re: RPM Of A Bullet In Flight

and that is why some HOT varmint rounds will sometimes "come apart" in midair. The jacket will rip and the core will disintegrate.
 
Re: RPM Of A Bullet In Flight

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tripwire</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rg1</div><div class="ubbcode-body"> Another way of thinking about twist is that for a 1/12 twist barrel the bullet will turn once every foot of travel or 300 revolutions in 100 yards or 900 complete revolutions in 300 yards. Not a lot of turns but it's doing it in a short time. A 1/9 twist barrel would spin the bullet 400 times in 100 yards. </div></div>

This....^

The bullet from a 1:12 twist turns 3 complete revolutions every yard it travels down range....and is slowing down the instant it clears the crown, although the rotation is slowing at a MUCH slower rate than the forward progess.

It's not a freaking tazmanian buzz saw coming out of the muzzle........ </div></div>



I don't know any quantitative data, but I've heard the same, that rotational speed doesn't slow down much, as velocity drops of. Figure a 1/12 twist at 2600 fps is rotating roughly 1/6 by the time the bullet slows to 1300 fps.
 
Re: RPM Of A Bullet In Flight

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MontanaMarine</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tripwire</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rg1</div><div class="ubbcode-body"> Another way of thinking about twist is that for a 1/12 twist barrel the bullet will turn once every foot of travel or 300 revolutions in 100 yards or 900 complete revolutions in 300 yards. Not a lot of turns but it's doing it in a short time. A 1/9 twist barrel would spin the bullet 400 times in 100 yards. </div></div>

This....^

The bullet from a 1:12 twist turns 3 complete revolutions every yard it travels down range....and is slowing down the instant it clears the crown, although the rotation is slowing at a MUCH slower rate than the forward progress.

It's not a freaking tazmanian buzz saw coming out of the muzzle........ </div></div>



I don't know any quantitative data, but I've heard the same, that rotational speed doesn't slow down much, as velocity drops of. Figure a 1/12 twist at 2600 fps is rotating roughly 1/6 by the time the bullet slows to 1300 fps.

</div></div>
The basis of Cheytac (now out of business) controlled spin. It was a way of putting deep groves in the bullet with the rifling that would slow the spin a lot more than natural decay to make it nose down over distance like a nice football throw.
 
Re: RPM Of A Bullet In Flight

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: VonReich</div><div class="ubbcode-body">just curious, but how did that question come along? </div></div>

It was one of those ... "well the bullet is spining about as fast as a football" in flight. And I said no way, it screaming out of there and has to be up in the tens of thousands of rpms. It was just a ... "neither one of us really knew what we were talking about" conversation .... LOL.
 
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