In article ,
says...
Ignoramus8862 wrote:

The momentum would be the same as the
rifle's recoil (which is painful), only transferring more energy
because of higher speed of the affected area of the vest, compared to
the speed of rifle moving backwards.

Is the bullet a rocket?
50% of the energy go into the bullet, the other half into the gun.
E = m/2 v^2. That is true for both the bullet _and_ the gun.


Only in the unlikely case that the mass of the bullet and
gun are equal, otherwise the energy will be divided in
inverse proportion to the ratio of their masses. In other
words, if the gun is 99 times as massive as the bullet, the
bullet will leave the barrel with 99% of the energy.

Ned Simmons