Breech block

A breechblock (or breech block) is the part of the firearm action that closes the breech of a weapon (whether small arms or artillery) at the moment of firing.

A way of closing the breech or chamber is an essential part of any breech-loading weapon or firearm. Perhaps the simplest way of achieving this is a break-action, in which the barrel, forestock and breech pivot on an hinge that joins the front assembly to the rear of the firearm, incorporating the rear of the breech, the and usually, the trigger mechanism. A breechblock is a separate component and is not a feature of the break-action. A breechblock must close against the breech for firing but be able to be retracted for loading or unloading or to remove a spent cartridge.

This article primarily addresses the matter of breechblock design, as opposed to the action, which relates more with how the mechanism is operated, even if the distinction is not always clear.

Usually referred to as a bolt rather than a breechblock, a rotating bolt is perhaps the most common variant. It is so called, because its operation is similar to a pad bolt or barrel bolt. The bolt slides in the receiver along the axis of the barrel and is rotated in the same axis to lock or unlock it against a closed breech. It is the basis for the bolt action, in which the bolt is rotated and retracted by an handle attached to the bolt. In some designs, handle (sometimes called a cocking handle) rotates to lock against a shoulder in the receiver or body of the firearm. This type of locking is usually reserved for low-pressure applications such as the .22 cal rimfire series. More often, the bolt locks closed with two or more lugs that operate like a bayonet mount. Multiple lugs permit a smaller degree of rotation to lock and unlock the breech. Most types are front-locking and have the lugs mounted near the breech face. A notable exception is the rear-locking system used in the Lee–Enfield.

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