A breech-loading gun is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel.
Modern mass production firearms are breech-loading (though mortars are generally muzzle-loaded). Early firearms, on the other hand, were almost entirely muzzle-loading. The main advantage of breech-loading is a reduction in reloading time—it is much quicker to load the projectile and the charge into the breech of a gun or cannon than to try to force them down a long tube, especially when the tube has spiral ridges from rifling. In field artillery, breech loading allows the crew to reload the gun without exposing themselves to enemy fire or repositioning the piece (as was required for muzzle-loaded guns) and allows turrets and emplacements to be smaller (since breech loaded guns do not need to be retracted for loading).