A rifle is a long-barreled firearm which imparts a high velocity to a small ballistic, called a bullet, by exploding a charge behind the bullet. (See figure.) A rifle is typically 4 kg, and the bullet is 10 grams. The velocity of the bullet upon leaving the barrel is 300 m/s. Take the system to mean the combination of rifle and bullet. Normally the rifle is fired with the butt of the gun pressed against the shooter's shoulder. Ignore the force of the shoulder on the rifle.
What is the recoil velocity of the rifle, that is, the velocity of the rifle just after firing?