Final answer:
An ordinary rifle recoils due to Newton's third law of motion as the gun exerts a force on the bullet and the bullet exerts an equal and opposite force on the gun. A recoilless rifle reduces its kickback by venting gases out the back but it is not safe to stand behind it due to high-velocity gases and debris.
Step-by-step explanation:
The recoil of an ordinary rifle occurs due to Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When a bullet is fired, the gun exerts a force on the bullet forward, and the bullet exerts an equal and opposite force on the gun, causing it to kick backward.
The recoilless rifle is designed with a vent at the opposite end of the barrel. This allows the gases produced by the firing to escape out the back, which cancels out a good portion of the recoil. Newton's third law applies here because, as the projectile is forced out one end due to the expanding gases, the escaping gases out of the back end provide a counter force.
It is not safe to stand close behind a recoilless rifle when it is fired because the high-velocity gases and possibly debris escaping from the rear of the barrel can cause injury.