Question

Explain therefore of a gun in light of Newtown third law of motion.

Answer 1

When a bullet is shot from a gun, the gun pushes the bullet with a force in forward direction (action) and the bullet pushes the gun with equal and opposite force in the backward direction (reaction). So the gun recoils. 

Answer 2

When a bullet is fired the gun recoil and move backward but the bullet go forward.