Recoilless rifles are guns that can shoot very large bullets or explosives without any recoil. This is important because Newton's First Law of Motion teaches that whenever something moves, there is a motion that goes the opposite direction that is just as strong. This is why guns have recoil. Whenever a gun shoots a bullet forward, the bullet going forward creates an equally strong movement backward. The heavier a bullet is, the more force needed to fire a bullet forward, and therefore the more force the bullet sends the gun backward. Recoilless rifles can fire very large bullets and explosives without hurting their users because they are made with a hole in the back of the gun so that the force from firing the gun goes behind the user, rather than towards the user's shoulder. People using recoilless rifles must be very careful that no one is behind them when they are shooting because the force going behind the gun, or the backblast, is powerful enough to kill a person.