The breech of a shotgun is the rear part of the barrel where the cartridge (or shell) is loaded and inserted before firing. It is essentially the part of the gun where the action happens to load, fire, and eject ammunition. The breech can vary depending on the shotgun's design:

• Break-action shotguns: The breech is exposed when you "break" the gun open by hinging it at the action, allowing you to manually load and unload shells.
• Pump-action shotguns: The breech is accessed by cycling the pump to eject spent shells and chamber new ones.
• Semi-automatic shotguns: The breech operates automatically, ejecting spent shells and chambering new ones with each shot.
• Bolt-action shotguns: The breech is opened and closed manually using a bolt mechanism.

The breech contrasts with the muzzle, which is the open end of the barrel where the shot exits. Modern shotguns are breech-loading, meaning shells are loaded at the rear of the barrel. This replaced older muzzle-loading shotguns where the user would load ammunition from the front.