Originally Posted by

**win71**

This occurs with spitzer (pointed) bullets that have either a flat base or a boat tail (tapered) base. It usually occurs in boat tail bullets at farther distances due to its higher ballistic coefficient of friction. If it's any help these projectiles are spinning at the rate of twist of a particular rifle however a rate of one revolution per 12 inches would be normal. A normal bullet speed exiting the barrel is around 3000 fps.

Given the fact that the bullet is pitching and yawing as soon as it exits the barrel and spinning really fast the theory is those radical movements settle down out past 100 yards or so and the bullet becomes more stable in flight.

The question is why does it do that?

This is pretty close to correct. However, the reason is in the way atmospheric drag works. Basically, there is a cruising speed for a bullet as it rips through the atmosphere. It has to slow down to that speed before its path will become stable. So, it has less to do with spin, and more to do with the atmosphere simply exerting less drag.

This is the equation.

http://en.wikipedia.org/wiki/Drag_%28physics%29
F is the force of drag,

P is the density of the fluid,[3]

V is the speed of the object relative to the fluid,

A is the reference area,

C_d is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car), and

The weird V is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).

A, C_d, and P don't change much during flight. The bullet's drag properties, and the air's thickness are pretty much just constants. The weird v is just to describe the direction it's traveling in, and it's not terribly important either because the drag will be about the same no matter what direction the bullet is traveling in.

The thing that does change is the bullet's velocity, V. Initially, right out of the barrel, when it's going really really fast, it's experiencing a lot of drag. Since velocity is squared in the equation, that means it isn't just experiencing a little bit more drag when it's going fast. It's experiencing exponentially more drag. Once the bullet slows down to its cruising speed, that drag will have diminished to the point where it's much more acceptable, and that should mean that the bullet's path remains more stable.

I'm going to post one more link that could be fun to read. It turns out that the cruising speed for bullets in water is so low that they're not even lethal. And, because water is thicker than air, they reach that speed much more rapidly than they do in air. I think it only takes about 14 feet.

http://kwc.org/mythbusters/2005/07/m...oof_water.html