After reading the replies of both the effects of the wind on stability and the 32-40 twist rates someone mentioned about the "Center of Pressure". I am really in over my head on most of this stuff but think that I can see where the center of gravity can be calculated and it is a meaningful measurement but the Center of Pressure? Can this actually be calcuated in the length of the bullet? 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. Is there somewhere that has more information on this. I think that you definitely want the center of pressure behind the center of gravity but can you determine that value?

Richard

rhbrink wrote on Oct 30^th, 2012 at 8:41am:


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.

Will do.

RB

When I post on the subject of external ballistics it is with the understanding that we're primarily concerned with schuetzen velocities - typically 1300 to 1500 fps.  Accordingly, our bullets are in the dread "transonic zone" from muzzle to target.

Consider:   

"When the velocity of a rifle bullet fired at supersonic  muzzle velocity approaches the speed of sound it enters the transonic  region (about Mach  1.2–0.8). In the transonic region, the centre of pressure (CP) of most bullets shifts forward as the bullet decelerates. That CP shift affects the (dynamic) stability of the bullet. If the bullet is not well stabilized, it can not remain pointing forward through the transonic region (the bullets starts to exhibit an unwanted precession  or coning motion that, if not damped out, can eventually end in uncontrollable tumbling along the length axis). However, even if the bullet has sufficient stability (static and dynamic) to be able to fly through the transonic region and stays pointing forward, it is still affected. The erratic and sudden CP shift and (temporary) decrease of dynamic stability can cause significant dispersion (and hence significant accuracy decay), even if the bullet's flight becomes well behaved again when it enters the subsonic  region. This makes accurately predicting the ballistic behaviour of bullets in the transonic region very difficult." 

Our sport is one of the most difficult shooting disciplines because bullets are in the transonic zone from muzzle to target.  Further, it is virtually impossible to accurately model bullet performance at Schuetzen velocities.  So, we do our best to avoid marginally stable bullets and rely heavily on trial and error for best performance.

JackHughs