Final answer:
The physics law that explains why cartridge cases end up with breech markings is Newton's Third Law of Motion. When a rifle is fired, the cartridge case imprints breech markings due to the equal and opposite reaction forces at play. Recoilless rifles minimize recoil by expelling gases out the back of the barrel, but it is still unsafe to stand behind them when fired due to the hazardous back blast.
Step-by-step explanation:
Understanding Recoil in Firearms
The physics law that explains why cartridge cases end up with breech markings is primarily Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. When a rifle is fired, the bullet is propelled forward through the barrel due to the expansion of gases following the ignition of the gunpowder. In accordance with Newton's law, the gun itself is pushed backwards with an equal force, which is experienced as the recoil. The rearward motion of the gun results in the cartridge case moving back with sufficient force to imprint breech markings.
The situation with a recoilless rifle is different. These rifles have a barrel that is open at both ends. When a recoilless rifle is fired, the bullet moves forward, and high-pressure gases are expelled out of the back of the barrel. By ejecting gases backwards, the backward force on the gun is largely negated, which means there is minimal recoil. However, due to the high velocity of these gases, it is not safe to stand close behind a recoilless rifle when it is fired, as the back blast can cause injury.
When cartridge cases obtain breech markings, it's a result of the momentum transfer and energy interactions within the firearm when the bullet is discharged. The recoil force and its effects, which include the breech markings on cartridge cases, are also influenced by the conservation of momentum and the energetics of the weapon's discharge process.