Rate of spin

[Clearmore]

I got to thinking about how fast a bullet is spinning when it exits the muzzle.  I was so surprised with the number I got that I had to do the math several times to see where I might have went wrong.

Assume a muzzle velocity of 2500 feet per sec. and a 1 in 10 twist rate.  This gives the bullet an astounding (to me at least) 180,000 revolutions per minute!  

2500 feet per second = 30,000 inches per second
30,000 inches / 10 inches for each revolution  = 3000 revs per sec.
3000 x 60 seconds in a minute = 180,000 revolution per minute

Maybe this is old news but I'm new at this stuff and that kinda surprised me!

[quickdtoo]

Now ya know why lightly constructed bullets fragment when they hit something! Figure the RPM of a .172 Vmax bullet from a .17hmr at 2500fps from a 1 in 9 twist..... :eek: And why hunting bullets cut such a devastating wound channel after they expand but stay together!!

[Nightrain52]

You don't see or hear much about it anymore but several years ago in the faster varmint rounds the bullet would disentigrate in a gray puff before ever reaching the target because the jacket wasn't heavy enough to hold it together.

[MINNESOTA DICK]

How to figure bullet RMP at the muzzle of a gun.

12" / rifle twist x velocity fps x 60 sec = bullet RPM

Example:

12 / 10 x 3200 x 60 = 230,400 rpm

[Donaldo]

Astounding is it not, just how fast they spin.  Now you know why cheap bullets don't shoot worth a hoot.  There is a reason benchrest shooters only shot the very best bullets.  Any little bit of imbalance at that rpm causes the bullet to do funny things on its way to the target.

[Lone Star]

Some may think that 180,000 rpm is fast - and it is - but think about the reality of a bullet's journey.  Shoot it at a target 100 yards away and the bullet has only completed about 400 revolutions by the time it gets there, about the same number of steps a small child would take walking to the target.  It doesn't go through an animal quite like a buzz saw either, making only a relatively few revolutions before stopping (confirmed by photos in gelatin).  The energy used to spin the bullet is only about 0.35% of the total energy of the bullet at the muzzle, so fast rates of spin don't realy reduce the bullet's velocity potential.

[bgjohn]

Some may think that 180,000 rpm is fast - and it is - but think about the reality of a bullet's journey.  Shoot it at a target 100 yards away and the bullet has only completed about 400 revolutions by the time it gets there, about the same number of steps a small child would take walking to the target.  It doesn't go through an animal quite like a buzz saw either, making only a relatively few revolutions before stopping (confirmed by photos in gelatin).  The energy used to spin the bullet is only about 0.35% of the total energy of the bullet at the muzzle, so fast rates of spin don't realy reduce the bullet's velocity potential.

 :shock: Yeah but...... :eek:
JM

[MSP Ret]

Thanks Lone Star, good info and rational thinking. It really put everything into perspective....<><.... :grin: