Basic Information
The diametric opposite of muzzle loading - the rounds are loaded from the back of the barrel through a moveable section known as the breech. This is now the normal way of loading a firearm and is generally faster and more efficient than muzzle loading as well as an essential function of any repeating weapon.
Breech loading is almost as old as firearms themselves as several designs of early cannon featured a removable breech section, however for centuries breech loaders struggled to achieve a tight enough seal to prevent exhaust gases escaping. At a minimum a failure to seal will cause a loss of muzzle velocity - more likely it causes injury to the operators and can damage the closures to a degree that parts of the breech are violently ejected and the weapon ruined. The Ferguson Rifle was a prime example of a good weapon spoiled by a poor seal, as was the far more widespread Dreyse Needle Gun.
Once the sealing problem was resolved the breech loading principle quickly proved superior to muzzle loading in all aspects and has superseded it in almost all roles. The main exception in modern times is the mortar, the majority of which remain muzzle loaded.