Bullet RPM

[204shooter]

I ran across a formula you all may be interested in. It really blew my mind.

Vel. x 720
----------- = Bullet RPM
Twist rate

 

[SHERMY]

If I got the math correct, a bullet leaving the muzzle @2800fps with a 12 twist barrel only completes approx. 280 revolutions before it hits a target 100yds away.
Correct me if I'm wrong.

 

[fivering]

If the twist is 1 in 12 inches, then there will be 300 revolutions in 100 yds,300 ft).

 

[Fred Bohl]

SHERMY

With a muzzle velocity of 2800fps and a 1 in 12 twist barrel your bullet spins at 168000 rpm or 2800rps. however air drag will reduce the forward velocity,say 2750fps average) and spin rate,say 2700rps average) so that your time of flight is about 0.109sec,300ft/2750fps) for about 294 rev,0.109sec x 2700rps)

 

[CatShooter]

SHERMY

With a muzzle velocity of 2800fps and a 1 in 12 twist barrel your bullet spins at 168000 rpm or 2800rps. however air drag will reduce the forward velocity,say 2750fps average) and spin rate,say 2700rps average) so that your time of flight is about 0.109sec,300ft/2750fps) for about 294 rev,0.109sec x 2700rps)

Rotational rate falls off much more slowly than forwards velocity... so the bullet will make more than 300 revolutions by the time it has traveled 100 yds

 

[Fred Bohl]

CatShooter

If the spin rate is holding up better than I estimated then yes you are correct, there will be more than 300 revolutions before 100 yards. The quick look research I did before answering did not have any magnitude data but did imply an initial sharp loss during stabilization. If you have a good reference that provides better data for our velocities and bullet shapes please post the title and author as I'm particularly interested in some real data in stabilization part of portion of bullet flight.

 

[CatShooter]

CatShooter

If the spin rate is holding up better than I estimated then yes you are correct, there will be more than 300 revolutions before 100 yards. The quick look research I did before answering did not have any magnitude data but did imply an initial sharp loss during stabilization. If you have a good reference that provides better data for our velocities and bullet shapes please post the title and author as I'm particularly interested in some real data in stabilization part of portion of bullet flight.

It is a well known fact - it goes back to studies at Picatinny Arsenal back in the 50's... maybe even earlier.


.

 

[gunsandgunsmithing]

Call me stupid but I can't see how a bullet that is spinning at a rate of 1 revolution per foot is going to turn more or less than 300 times in 300 feet of flight regardless of velocity. Velocity will affect time of flight and rpm proportionatly---Mike

 

[CatShooter]

"... Call me stupid..."

OK you're Stup... naw.

"... but I can't see how a bullet that is spinning at a rate of 1 revolution per foot is going to turn more or less than 300 times in 300 feet of flight regardless of velocity. Velocity will affect time of flight and rpm proportionatly---Mike

Lemme 'splain.

A bullet that has a 12" twist means that it makes one full turn for every 12",one FOOT) of forward motion.

So it leaves the barrel, and it will turn once, when it has traveled one foot!

It will have turned 10 times when it has traveled ten feet... there are 300 feet to 100 yds, so ergo - it turns 300 times to get there.

Now... there is a lot of resistance to forwards motion of a bullet. The energy to form and keep suspended, the sonic cone coming from the front of the bullet, sucks up a lot of energy, so forward velocity decreases rapidly.

But there is very little air friction in the rotational plane,in the beginning of flight), so the rotational rate slows down at a much lesser rate... so the bullet will rotate more than 300 times by the time it has traveled 300 feet.

The amount of rotational disintegration is not predictable, because it depends on many factors - the number and depth of the grooves, the angle of attack of the air flow to the angle of the rifling, how deep the rifling is, and whether the rifling is standard, or ratchet, etc.

 

[gunsandgunsmithing]

Thank you for the good explanation. I now see why.---Mike

 

[rbertalotto]

And once the bullet hits something like a prairie dog, it keeps spinning at thousands of RPM for a few nanoseconds.........See exploding prairie dog for example.......This rotational energy has to go somewhere.........Although this energy is very small I bet it has an effect on something small like a PD.......

 

[CJ6]

But wait,If the muzzel is 100 yds,300ft.)from the target and you have a 24 in. barrel you will have 301.5/6 revolutions asuming a 308 case

 

[CatShooter]

But wait,If the muzzel is 100 yds,300ft.)from the target and you have a 24 in. barrel you will have 301.5/6 revolutions asuming a 308 case

No... you obviously missed something BIG!!


.

 

[rbertalotto]

Actually it's not all that small. If my math is correct a 30 caliber 100 grain bullet rotating at 100,000 RPM has over 100 ft/lbs of rotational energy. If the bullet doesn't exit, that energy will be expended in the target.

WoW! No wonder those big fat PD really come apart when I hit them with a 6mm - 55grain Nosler coming out of my 1-7 Twist 6mm Ackley, traveling at some 4000 FPS! Mass X Velocity squared = Energy ...PLUS rotational energy.........BOOM