[Oztrich Boy]

In the kinematic mechanism of trauma, it is the square of the velocity that is significant not the mass of the projectile.

That's energy, (a measure of the potential for damage) not momentum, (which causes the movement).

I made a calculation 10 years ago (last time this subject was flavour of the day). IIRC if only 10% of the bullet energy had gone into moving brain tissue foward (as shown on the Zapruder film) that would have given a rearward momentum (from large mass at low velocity) to the skull of FOUR times the previous momentum from the bullet (tiny mass at high velocity).

This agrees with what Zapruder shows - a small forward movement in framw 314, followed by a larger rearward momentum in th3e following frames.

[Ophiucus]

The energy is also directly related to the force acting upon the head to change it's momentum. The velocity has a much greater significant effect than the mass.

Moving brain tissue forward within the head would never produce a backward motion unless the source of the movement originated in the head - like some sort of weird bomb. The bullet impacting the head imparts motion to the entire head. Hit the head and brain and skull move together in the diretion of the hit.