phy-recoil

(a) Gain in momentum of bullet = (20/1000) x 200 kg.m/s = 4 kg.m/s
Hence, gain in momentum of gun = 4 kg.m/s (Conservation of Momentum)
Recoil velocity of the gun = 4/4 m/s = 1 m/s

(b) Thrust on bullet = rate of change of momentum = 4/0.2 N = 20 N
By Law of Action and Reaction (Newton's Third Law),
Thrust exerted on shoulder of man = 20 N

(c) By including the exhausted gas, the change in momentum is larger than before. Hence, the recoiling force also becomes larger.