In: Physics
During the firing of a bullet, the total momentum of the system(gun + bullet) is conserved as there is no external force acting on the system. As the bullet gains some momentum when it is fired , the gun moves back a little in the opposite direction to make the total momentum the same as it was initially( initial momentum = 0). This is called the recoil of a gun.
Therefore ,the magnitude of impulse(change in momentum) of both bullet and gun will be the same but they will be in opposite directions only.