rhbrink wrote on Oct 30
th, 2012 at 8:41am:
I can see in my own little way that the pressure might be different between the top of the bullet base verses the bottom depending of the position of the powder even the different types of powder and primer.
The center of pressure is an aerodynamic term. It applies when the bullet is in flight. If the bullet yaws, pressure will change all across the side of the bullet.
The bullet's mass is distributed all along the bullet, but for the purpose of balance, it can be thought of (and calculated) as being concentrated at one point. The same goes with pressure.
For aerodynamic stability, you want the center of pressure (CP) behind the CG. When flying straight into the wind, there is no net pressure along the sides, only onto the center of the nose. As the bullet (or the wind) yaws, the pressure profile on the side changes and there is a net push. If the CP is behind the CG, it will rotate the bullet back into alignment.
An arrow is this way. The big tail fin ensures the pressure will have a large area to act on at the rear, thus keeping the CP aft.
Most bullet shooting these days is supersonic. The CP shifts greatly varying with the speed. Calculating the CP is not trivial, and it's more difficult if it changes with speed.