Breechblock

The firearm action more fully refers to the mechanism by which the operator actuates the opening and closing of the breech.

A breechblock must close against the rear of the breech for firing but be able to be retracted or otherwise moved for loading or unloading or to remove a spent cartridge.

[notes 1] 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.

Multiple lugs permit a smaller degree of rotation to lock and unlock the breech.

Rotating bolts can be adapted to automatic or semi-automatic designs and lever or pump actions.

This action is commonly achieved by a slot cut in the carrier that engages a pin through the bolt perpendicular to the axis of the barrel.

Straight-pull bolt-action firearms do not require the operator to rotate the cocking handle to cycle the action.

The breechblock is well supported by the receiver within which it slides and the mechanisms for opening and closing the breech do not have to act to any extent against the forces generated on firing.

The breechblock is hinged parallel to the axis of the barrel and swings away to the side to expose the breech.

The breech is opened by rotating it a fraction of a full turn, until a hole through the breechblock aligns with the bore.

The breechblock design as has been called a falling or tilting block but omitting the role of the hinge can lead to ambiguities.

To unlock the breech, a slide or carrier moving rearward uses a wedge or ramp-like arrangement acting on the sides of the breechblock to tilt it up at the rear and lift it clear of the locking shoulder.

The carrier or slide can be operated by lever or pump actions or by gas, for automatic and semiautomatic fire.

The tilting action is achieved without a separate breechblock carrier but by the design of the cocking handle.

[25] The breech is opened by the breechblock moving in-line with the axis of the barrel and is locked in the closed position by an obstruction such as a cam, wedge, pawl or levers.

[30] The straight-pull Heym SR 30 uses ball bearings for locking, similar to an air fitting connector.

[32][33] Blowback actions use an in-line breechblock in which the breech is never locked and is held closed by spring tension alone.

It is common in semiautomatic rifles and pistol chambered for .22 cal rimfire cartridges and many submachine guns.

The slide contains the breechblock and is initially locked to the barrel so that the combined assembly move together.

A short movement trips the mechanism to unlock the barrel from the slide assembly, allowing the breech to open.

This type of floating configuration and recoil operation is not confined to pistols and may be found in machine guns and auto-firing cannons.

The Ross rifle Mk III is a straight-pull design that has multi-thread locking lugs.

The Ferguson rifle used a tapered screw plug inserted perpendicular to the axis of the barrel.

Illustration of some firearm breechblocks
A typical break-action , double-barreled shotgun
Springfield Model 1888 , breech open
An L1A1 disassembled to show the tilting breechblock. The breechblock fits inside the slide. There is a chamfer (A) on the top rear of the breechblock and a corresponding internal chamfer in the slide. For the chamfer (B), there are corresponding inclines machined inside the slide. A return spring acts on the rod at the rear of the slide, pushing the slide forward. As the breech closes, the slide acts on the chamfer (A), to push the breechblock down into a recess in the receiver and lock the breech. Upon firing, the gas piston pushes the slide rearward. The ramp within the slide acts on chamfer (B), tilting the rear of the breechblock up to disengage it from the recess and unlock the breech. [ 22 ]
Animation of simple blowback operation.