In terms of conservation of momentum why does a gun kick when it's fired?

Answer 1

In the case of a semi-automatic pistol, expanding gases push everything in the barrel apart, propelling the bullet forward down the barrel, and the slide back toward the user. The slide continues its momentum back until it cannot go any further (At this point, it begins chambering the next round), the slide is transferring it's kinetic energy into the frame of the gun. The gun pushes back into the hand, and the hand torques back and up, experiencing "kick". In the case of limp-wristing (not holding it properly/tightly enough), or powerful magnum rounds, when the hand cannot torque anymore, the momentum will force the elbow or shoulder to bend, creating that infamous sight of a huge gun flying upwards after being shot. Any excess energy at this point has been absorbed by your body (or if the recoil was extreme, forced the user to step back to avoid falling). Finally a spring pushes the slide forward, sending the gun back (Though very small compared to the actual shot, it's noticeable.)

Bullets are very small but move at incredible speeds, rendering them lethal.

Conservation of momentum - The momentum (m*v) of the small object (bullet) is equal to the momentum of the gun recoil.

Kinetic Energy - The kinetic energy (1/2 m*v*v) of the small object (bullet) is about 350 times as much as the energy of the gun recoil. About 99.7% of the gun powder energy is used to propel the small object (bullet). About 0.3% of the gun powder energy is used to propel the gun back (recoil). Data: 22LR, 40 gr, 1255 FPS, 32 oz S&W handgun.

The reason the kick back doesn't kill the user is because the gun is significantly more heavy, thus slower moving and easier to stop. The smaller the gun, the more painful the kick. The larger the gun, the slower it moves, and the more time it takes to absorb the momentum, which can be good or bad depending on the situation. Though not frequently noticed, smaller guns can potentially be returned to their target after recoil significantly quicker than larger guns, because in smaller guns the momentum is being stopped more suddenly.