Final answer:
The chamber end of the barrel of a firearm is referred to as the breech, which is where the cartridge is loaded before firing.
Step-by-step explanation:
The chamber end of the barrel is called the breech. This is the portion of the barrel where the cartridge is inserted before it is fired. The breech is an integral part of the firearm which is critical to its function, because it is where the bullet begins its journey down the barrel after the gunpowder is ignited. The other terms provided, such as butt, backend, and freebore, refer to other parts of a firearm or to other concepts entirely. The butt is the rear end of the gun which the user holds or shoulders, the backend is not a standard term in gun terminology, and freebore refers to a section of the barrel in front of the chamber where the bullet travels before engaging the rifling.