A gun recoils when a bullet is fired. This is because the initial momentum is zero as they are both stationary relative to each other.According to the principle of conservation of momentum the final momentum of the system must also be zero. So the momentum of the gun and the bullet must be equal and opposite for their sum to be zero.The difference between a collision and an explosion is that in a collision at least one of the objects has a non-zero momentum, whereas in an explosion all the objects are initially joined and initially have zero momentum relative to each other.