Question

When a bullet shot from a rifle,the rifle required back .why?

Answer 1

Bullet fired -action, recoil of the gun-reaction.
when a bullet is fired from the gun, the force of the internal explosion in the gun fires the bullet forward as a projectile. This is considered as action. The sudden recoil of the gun caused by the internal explosion seen hence is the opposite force applied by the explosion. Thus Newton's third law is applied.

Answer 2

Gun recoil back when a bullet is fired from it due to principle of conservation of linear momentum.