recoiling of rifle is an eg. of momentum between the bullet and the rifle. in this we will equal the momentum of the bullet of mass m1 and velocity v1 with the rifle of mass m2 and velocity of recoil v2 acc. to newton's 3rd law of motion.
which implies, m1v1=m2v2
m1v1/m2=v2
hence,we can find the velocity of recoil of rifle.