Rate of Twist & RPM

[choprzrul]

So my mind starts wondering off of work today and I start to think about the forces at work on a bullet that try and tear it apart. I start thinking about how fast a bullet spins in revolutions per minute (RPM). The numbers in my head seemed extreme, so I pushed the thought away until now. Here is my rough parameters and calculations:

1 in 12" rate of twist barrel
3200 fps muzzle velocity
333yd (1000 feet) target distance
.33 second flight time

The bullet does 1000 full revolutions in its flight to the target in 1/3 second. This is equivelent to 3000 revolutions in 1 second, or 180,000 RPM.

This seems completely over the top extreme rpm to me. Where am I wrong in my calculations? If, roughly, the calcs are correct, how do bullets keep from flying apart in flight???

Can't wait until morning to read responses.

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[Boberama]

It would be 192000 RPM at the muzzle.

But the bullet would slow down, so you can't actually calculate how much RPM it has at the target.

.220 Swift bullets did more damage to steel plates and mules than .30-06s.
According to "The Rifle in America".
The author credits it to RPM.

[choprzrul]

But still the question remains, how does a bullet manage to stay intact @ 150,000 + rpm? Is there anything else that you can think of that rotates at that speed?

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[Mickey D]

But still the question remains, how does a bullet manage to stay intact @ 150,000 + rpm? Is there anything else that you can think of that rotates at that speed?

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Engineering.

[yakmon]

the weight-moment of the bullet isn't much, though, consdering the arm of the bullet, for example a .223 rem bullet, .224 diamater, the arm is only .112, and the weight isnt a while bunch, either. 55 grains is only 0.00785 of 1 lb.

[choprzrul]

The bullet's velocity slows in flight due to resistance from the air; so does the rotation slow proportionatily? Picturing bullet impact on something like an elk, the bullet actually drills into the animal.

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[bussda]

Physics speak: The centripetal force exerted on the bullet is less then the tensile strength of the copper jacket. Bullets used over a certain muzzle velocity need to be jacketed or they fall apart in flight.

[TKM]

Lightly jacketed varmint bullets can and occasionally do disintegrate in flight. Usually when pushed to the upper end of rifle velocities, near the 4000 fps neighborhood.

http://forums.gunsandammo.com/forum/general-shooting-information/22-250-reload-ammo-disentigrating-between-175-200-yards

[tzahoy]

But still the question remains, how does a bullet manage to stay intact @ 150,000 + rpm? Is there anything else that you can think of that rotates at that speed?

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I spoke with a guy working on development of miniature electric turbine generators used in drones. They scavenge pressure from the jet engine to spin the turbine and make electricity to power the onboard electronics. The turbine generators are about 1/2" diameter and spin 500,000 rpm and generate 1 kw of power. They have a planned development roadmap to 1,000,000 rpm.

Makes 150,000 rpm seem slow!

[BigFatGuy]

The bullet is solid, small, and made of very strong materials. These things all help it withstand the internal pressures of rotation.

[CaliforniaLiberal]

So just imagine the .223 moving through skin and into muscle, starting to turn a little sideways, maybe flex a little and BOOM all that rotational energy is released in an instant, fragments go flying off here and there, bigger pieces tumble.... I guess that's part of how that little caliber can make big nasty wounds.

[choprzrul]

So just imagine the .223 moving through skin and into muscle, starting to turn a little sideways, maybe flex a little and BOOM all that rotational energy is released in an instant, fragments go flying off here and there, bigger pieces tumble.... I guess that's part of how that little caliber can make big nasty wounds.

Well, yeah, but the effect of a larger diameter object spinning at roughly the same speed, would tend to have an even greater effect. I would think something with over double the weight and 50% greater diameter would really get squirly.

I wonder what effect the rifling imprint left on the bullet has on the bullet in flight. The imprint would match rate of twist and I would think it would help overcome the tendency to stop rotating in flight

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[UBFRAGD]

In Arnolds "High Power Rifle Accuracy", p. 88, the formula seems to be:

rpf (revolutions per foot) x fps x 60 (seconds to minutes) = rpm at muzzle exit.

rpf on a 1-10" twist rifle, converted to feet, would be 1.2 revolutions per foot

The next paragraph describes how to calculate how many revolutions the bullet makes to impact. Interesting stuff.

I might have to take a remedial adults' math class soon.....

[EBR Works]

I'm aware of a project to create an optically guided "fire and forget" .50 BMG projectile that contains electronics that must endure these forces. Pretty amazing engineering....

[Desert_Rat]

180,000 rpm ain't all that fast. even a turbocharger in tractoe engines will turn at upwards of 130,000 rpm.

here's one for ya.

If a top fuel dragster runs the 1/4 in 4.4 seconds and rund 9500 rpm the entire way. How many revs did the engine make during the run?