Question

why does gun recoils

Answer 1

Bullet fired from the gun

Initially when no bullet is fired the net momentum of the bullet-gun system has zero momentum. When bullet is fired the magnitude of the velocity of the bullet in forward direction is equal to the magnitude of the velocity of gun in the backward direction. The direction of velocity is in opposite direction therefore the net momentum again comes out to be zero and hence the conservation of momentum. This is the reason why recoil of gun is observed.