Seems to me bullet deformation is also going to play an important role in deflection, too. The more distorted the nose of the bullet, the greater the forces pushing it off the side becomes.
Shinbone,
This is definitely true, as is John's mention of the "javelin" shape of expanded monolithics--though the phenomenon is not limited to monos.
The longer the shank of an expanded bullet, the more it tends to penetrate in a straight line. There's an excellent explanation of this in the A-Square reloading manual, ANY SHOT YOU WANT, published in 1996. It uses the principles of physics, and several illustrations and photos of bullets. One of the more telling quotes: "Conservation of Angular Momentum is key to the performance of a soft point bullet, If the petals expand too far, it will reduce the the rotational velocity, and the bullet follows a wildly erratic path in the animal." Which is why A-Square designed its Dead Tough softpoint to expand a relatively small amount, much like monolithics, "for high weight retention AND retention of its spin and its straight flight path."
