How Many Revolutions Does a Bullet Make?

[HGM22]

I'm interested in knowing how many revolutions a bullet makes in some distance. For example, does a 1:7 AR15 barrel cause the bullet to rotate 360 degrees every 7 inches in flight? I've always imagined bullets spinning quite fast, but is that really the case?

 

[Longhorn 76]

You are correct.

 

[IlikeSA]

When it exits the barrel it should be doing it's twist rate, assuming a few things.

1: It is sized right for the barrel
2: It does not slip off the rifling

As it travels, I believe it slows down in twist and speed due to air resistance. The speed is measurable but the twist...I don't know.

 

[CZ9shooter]

I've done some math in the past just out of curiosity. IIRC, I was getting some numbers in the hundreds of thousands for rpm.

 

[Drail]

In the words of George W. - "I believe your figures are in-acc-u-rut"."

 

[silicosys4]

If I do the math,
1 in 7 twist, goes 1.4 +/- revs per foot.
3000 fps at the muzzle means at the muzzle, the bullet is going 3000fps x 1.4 revs per foot x 60 seconds,
= over 250k rpm at the muzzle.
I think that's right

No wonder some bullets will literally fly apart if driven too fast.

 

[rcmodel]

Air drag slows the bullet much faster then it slows the RPM at long range though.

There is air friction over the nose constantly slowing it down.

But the bullet itself is in a relatively frictionless boundary layer behind the bow shock wave, so there is very little air friction to slow the spin down.

rc

 

[General Geoff]

Bullets do spin very fast. See the various 'bullet fired into ice' youtube videos.

 

[USAF_Vet]

It's the twist at the muzzle as the bullet exits that dictates the RPM, correct? Most modern guns has a steady twist rate from chamber to muzzle, but I've seen some older guns that utilize a progressively tighter rate of twist where it's spin is slower at the chamber and spin increases as it travels down the barrel. Was/ is there any benefit to this, and does it effect spin rate of the bullet as it exits that barrel?

 

[1911 guy]

Gain twist was popular in some muzzleloading rifles. As the relatively slow charge burned and began moving the projectile (usually patched round balls rather than conical bullets), there was less spin imparted and slightly more velocity was gained. As the ball accelerated, it began to spin faster as both the velocity and twist rate increased.

Gain twist fell from popularity both because any effect it may have was both negligible compared to constant twist barrels and the effect was only measurable in any amount at all with long barrels, large charges and large bore diameters.

You can still buy a gain twist barrel, but most people who do are merely buying because it accurately reflects the prototype of the rifle they're building. I'm not aware of any off-the-shelf muzzleloaders currently made with gain twist barrels.

 

[Jim Watson]

Bullets spin fast, but they also travel fast.
If you have a 12" twist barrel (common in .308 and also .223 before the Army went nuts) then the bullet is turning one revolution per foot of travel. Shoot at 100 yards, 300 feet, the bullet will have made 300 revolutions by the time it hits the target.

 

[desidog]

Gain twist was popular in some muzzleloading rifles.

The most recent production rifle i recall with Gain Twist rifling was the Model 1891 Carcano - i'd like to see a comparison between an unaltered rifle and a Truppe Speciali.

 

[rcmodel]

The S&W Model 460XVR employs 1:100/1:20 “gain twist rifling”.

rc

 

[Jim Watson]

There are a couple of BPCR barrelmakers doing traditional gain twist. Paper patch shooters say it gives good "confetti" as it cuts the paper off the bullet, leaving no kite tails to deflect the shot.

Bartlein will give you any twist you want any place down the barrel.

 

[230RN]

If I do the math, 1 in 7 twist, goes 1.4 +/- revs per foot.
3000 fps at the muzzle means at the muzzle, the bullet is going 3000fps x 1.4 revs per foot x 60 seconds,
= over 250k rpm at the muzzle.
I think that's right

That should be 1.7 revs per foot: 12 ÷ 7 = 1.71428. Here's your reality check: If it were 1 in 6" twist, it would be 2 revolutions per foot.

Most varmint rifles put their bullets out at around a quarter-million RPM. The .220 Swift was known to have its bullets fly apart from the spinning, leaving a trail of blue-ish smoke to the target. Or halfway to the target, anyhow.

It's the twist at the muzzle as the bullet exits that dictates the RPM, correct? Most modern guns has a steady twist rate from chamber to muzzle, but I've seen some older guns that utilize a progressively tighter rate of twist where it's spin is slower at the chamber and spin increases as it travels down the barrel. Was/ is there any benefit to this, and does it effect spin rate of the bullet as it exits that barrel?

Those are called "Gain Twist" rifles/barrels. Most of the time they don't work out too well because the additional engraving on the bullet distorts it too much, as well as reducing the gas seal of the bullet in the bore. The spin depends on the last rifling the bullet "saw" as it left the muzzle. The distortion wasn't that much of a problem with patched bullets or round balls.*

It is the velocity coming out of the muzzle at a given twist rate which determines the RPM. Faster muzzle velocity = faster spin.

Now as to what twist is best for a given bullet, look up "the Greenhill Formula," which is an approximation (a guideline) for the probable best twist to start with for a given bullet.

Terry, 230RN

* There's always a lot of experimenting going on, including the concept of "tapered" bores which had their diameters reduced as the bullet went down the barrel. The bullets were flanged, so the flange would collapse as the bore diameter got smaller. An example is the 14/9 mm Gerlich cartridge, where the bore went from 14mm to 9mm at the muzzle. (Slip that one in next time there's a "9 versus .45" debate ! )
From Datig's "Cartridges for collectors:

 

[Reloadron]

We just had a thread on this very line of topic a few weeks ago. Also see 230RN's post with regard to "the Greenhill Formula". Interesting stuff. The basic formula is relatively simple.

Ron

 

[jmorris]

http://www.schuemann.com AET barrels are gain twist as are SVI barrels.

 

[4thPointOfContact]

Ancient history now, but still relevant....

When the Black Talon was the subject of so much hysteria, one hysterical reporter was saying that it was "... practically a buzz-saw as it rotated at One Hundred Thousand RPM through flesh and bone..." ((I don't remember the actual quote or numbers but it was pure hysteria)).

How many RPM a projectile is going is almost irrelevant when it comes to terminal ballistics. The majority of projectiles are going to penetrate anywhere between 12 and 24 inches before coming to rest. That means a .55 Magnum EagleClaw BuzzSaw fired from a barrel with 1:36 twist rate is going to rotate anywhere from 1/3rd to 2/3rds of a turn and a .18 Super Velociraptor Special with a 1:6 rate gets 2 to 4 revs before coming to a dead stop. Far enough to open up 'cutting petals' but hardly a buzz-saw.

 

[CZ9shooter]

That sounds about right. In some of the super slow-mo gel tests out there we can see the rotating pedals. It looks to me like most handgun bullets get maybe one full turn after entering the media. Rifle bullets maybe just a little more.

 

[Curator]

Bullets do continue to rotate after being stopped by a medium. I have seen bullets spinning after going through sand bags and losing forward momentum. Bigger bullets do this much more than small, light bullets.

 

[danez71]

Bullets spin fast, but they also travel fast.
If you have a 12" twist barrel (common in .308 and also .223 before the Army went nuts) then the bullet is turning one revolution per foot of travel. Shoot at 100 yards, 300 feet, the bullet will have made 300 revolutions by the time it hits the target.

^^^^^^
And this puts in perspective.

It might spin 200k+ RPMS....... IF it was in flight for a whole minute.

 

[JohnKSa]

They spin very fast. Here's a plot that provides some insight.

http://thefiringline.com/forums/attachment.php?attachmentid=37995&d=1226145787

 

[Jim K]

If a bullet rotates at one turn in 12 inches, and the average body is 12 inches thick, a bullet rotates one turn in a body. "Buzz saw? Only in the fevered brain of an ignorant "all the news I can make up" broadcaster.

Jim

 

[Black Butte]

Here is the correct calculation which assumes a one-in-seven twist and a muzzle velocity of 3000 feet per second.

(3000 ft/sec)*(12 in/ft)*(1/7 rev/in)*(60 sec/min) = 308,571 rev/min

See how the units of feet, inches, and seconds cancel to yield the correct units of revolutions per minute (RPM).

 

[Blade First]

"It might spin 200k+ RPMS....... IF it was in flight for a whole minute." -- danez71

That'll get you an 'F' in Physics 101; and it's RPM...revolutions per minute...not revolution per minutes.

Further: http://www.diffen.com/difference/Centrifugal_Force_vs_Centripetal_Force