Supersonic Bullet Shards?

[Brian Albin]

Do the rotating shreds of an expanded bullet have supersonic tip speed even though the bullet's forward speed is subsonic?
Somebody must have worked this out by now; if not Dr Franklin Mann, then perhaps Aberdeen proving ground or Frankford Arsenal.
　
Erik Ohlen makes a hollow pointing pin with a square cross section to produce an expanded bullet with four cutters instead of a smooth circular perimeter. I am wondering if these cutters are producing sonic booms, sending out shock waves as the bullet travels through Deer lungs.
　
Here are comparison pictures of a bullet made with the usual round pin and one with the square pin from Mr Ohlen's web site http://www.hollowpointmold.com
179120

[Ballistics in Scotland]

I am pretty sure that a lead alloy bullet can't produce increased velocity in its fragments, and that there is no functional difference between a round and four-petal expanded bullet. On the other hand it is possible - just - that fragments of a more elastic metal might end up faster than the bullet.


179144

This is a Lee-Metford bullet photographed with an electric spark in a corridor of Imperial College by Professor Boys, in 1893. It seems a perfectly reasonable thing to do. You can see a pressure-wave just like the bow-wave of a ship, and it is important to understand that it doesn't get paid out lengthwise, like whiskers growing. If you draw a line at right angles to any point on that wave, it will point through the place in the bullet's path where that bit of pressure.

Below the bullet you will see a bulging wave where the bullet has hurled fragments of the lead wire into motion. They have passed the bullet's own pressure wave, indicating that they have acquired higher velocity. But I believe this is because the wire and the jacket metal are elastic. If a tennis ball and racket were totally inelastic, you could drive the ball no faster than you could move the racket. But as they are elastic, it will go much faster.

[tazman]

The outside edge of the bullet would have to be moving rotationally faster than the speed of sound(approximately 1100fps) in order to shed fragments moving that fast. I am going to round off the numbers for ease of understanding.
Lets assume a 357 diameter bullet. The circumference is 1.12 inches.
Lets assume a fast twist 9mm at 1 in 9 inches. Velocity to be 1000fps. This gives us 1333 revolutions per second.
Multiply 1333 times the circumference(1.12) gives the total inches rotated each second(1493).
Divide that number by 12 to get the speed in fps(124fps) at the outside edge of the bullet. This number is significantly below the speed of sound.
Changing the twist rate, bullet size, or velocity will only change the number proportionally so there is no way a subsonic bullet will shed fragments above the speed of sound.
I think we can safely assume the bullet rotation will slow as it expands due to the drag involved.

[Brian Albin]

Quote: "I am pretty sure that a lead alloy bullet can't produce increased velocity in its fragments"

I did not state my question well.
I am wondering if the rotational speed of the bullet is greater than it's linear velocity. I am not thinking of bits breaking off the bullet and becoming separate projectiles, I am asking about those strips of bullet metal hanging off the sides of an expanded bullet.

If out at the tips of these pieces of metal the speed as they are rotating is supersonic, then a shockwave would I think be sent out from the bullet into the animal. A hollow point pistol bullet might do this if it were made with the square pin, as this would create four petal shaped bits of metal. The round disc with it's smooth edge created by the round pin may have a supersonic edge if the bullet is rotating fast enough, but there would not be much of a shape present to set up shock waves.

Brian

[Brian Albin]

Thank you, for the math Tazman. I thought the formula would be simple, but I could not think what it would be.
I had not guessed the speed would be nearly that slow. So that answers that.

Brian

[35remington]

Yes. The rotational energy and velocity of a bullet is much, much lower than its forward energy and velocity.

[JSnover]

Even at supersonic speeds, I'd expect it to lose velocity so quickly following impact that it's subsonic by the time it's expanded.