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Illustration of some firearm breechblocks Firearm actions.svg
Illustration of some firearm breechblocks

A breechblock (or breech block) is the part of the firearm action that closes the breech of a breech loading weapon (whether small arms or artillery) before or at the moment of firing. It seals the breech and contains the pressure generated by the ignited propellant. Retracting the breechblock allows the chamber to be loaded with a cartridge.


Breechblocks are categorised by the type or design of the mechanism by which it is locked or closed for firing. The firearm action more fully refers to the mechanism by which the operator actuates the opening and closing of the breech.


A typical break-action, double-barreled shotgun ShotgunAction.JPG
A typical break-action, double-barreled shotgun

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 a hinge that joins the front assembly to the rear of the firearm, incorporating the rear of the breech, the butt 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 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.

A breechblock is not a component of designs with multiple or moveable chambers such as revolvers, harmonica guns, Kalthoff repeaters, Kammerlader rifles [1] or, split breech designs. [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.

Rotating bolt

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, the 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.

Rotating bolts can be adapted to automatic or semi-automatic designs and lever or pump actions. In these cases, the bolt is held by a bolt carrier. With the breech locked, an initial rearward movement of the bolt carrier causes the bolt to rotate and unlock. Similarly, when closing the breech, the final forward movement of the carrier causes the bolt to rotate and lock the breech. 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. It is a type of linear cam.

Straight-pull bolt-action firearms do not require the operator to rotate the cocking handle to cycle the action. Some straight-pull designs may use a rotating bolt but other breech-locking mechanisms can be employed.

Sliding block

The breechblock in a sliding block slides across the face of the breech to close it. The sliding action is perpendicular to the axis of the barrel. When the breechblock slides down to expose the breech, it is referred to as a falling-block, as used in the Sharps rifle. A sliding block is common in artillery. A vertical sliding block rises and falls while an horizontal sliding block slides to one side. It is a strong design. 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.

Side-hinged breechblock

A side hinged breechblock is used in the Snider-Enfield. Other firearms using this type of breechblock include the Warner carbine, the Joslyn rifle and the Tarpley carbine. The breechblock is hinged parallel to the axis of the barrel and swings away to the side to expose the breech. Firing force is contained by the rear of the breechblock bearing on the receiver. [14] [15] [16] [17]

Rotating drum

A rotating drum breechblock or rotary breech consists of a cylinder which rotates on an axis offset from the barrel. It is also known as a cannon breech because of association with some cannon designs. A longitudinal cut-out section or eccentric hole provides access to the breech. Rotating the cylinder then closes the breech. The geometry is not unlike the cylinder of a revolver. In the M1867 Werndl–Holub, the cylindrical breechblock is retained at the rear by the receiver. Mating faces can be profiled (ie not faced perpendicular to the axis of rotation) so that the breech tends to seal or fit more closely as it is closed. [18]

the Nordenfelt eccentric screw breech is a variation of this design. Instead of holding the breechblock between the breech at one end and the receiver at the other, in this design the breechblock is threaded around its circumference and is screwed into the breech until it meets the rear of the chamber. The breech is opened by rotating it a fraction of a full turn, until a hole through the breechblock aligns with the bore. The Magnum Research Lone Eagle pistol is a single-shot pistol chambered for rifle cartridges that also uses a rotating drum breechblock. [19]

Trapdoor breechblock

Springfield Model 1888, breech open Springfield Trapdoor breech open.JPG
Springfield Model 1888, breech open

Commonly associated with the Springfield rifle, the breechblock is hinged above the breech face and lifts up like a trapdoor to expose the breech. The breech is locked by a catch operating at the end of the breechblock furthest from the hinge. It is similar in principle to a break-action.


A rolling-block can be described as a quadrant which is hinged below the breech. The quadrant rotates through approximately 90° to provide access to the breech or close the breech. In the closed position, a number of different devices can be used to lock the quadrant and prevent it from opening. In the Remington Rolling Block rifle most closely associated with this type of breechblock, the hammer also has a quadrant which cams behind the breechblock and locks it. [20]


Initially used in the Peabody rifle, it saw more widespread use in the Martini–Henry and the subsequent Martini–Enfield. It employs a breechblock with a rear hinged falling block design, in which the breech is opened by permitting the front of the breechblock to drop down while pivoting on its hinge. [21] Firing force is transmitted through the knuckle of the hinge and does not act directly on the hinge pin. The breechblock design as has been called a falling or tilting block but omitting the role of the hinge can lead to ambiguities. It is also used in the Krag–Petersson rifle.

Tilting block

An L1A1 dissembled 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. L1A1 receiver and components.jpg
An L1A1 dissembled 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.

As a tilting breechblock closes on the breech, it is tilted up at the rear but it drops into a recess at the end of its forward travel - thus locking the breech closed. Firing forces are transmitted to the locking shoulder at the rear of the recess. 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 breechblock is then pulled rearward by the slide or carrier to expose the breech. In the closed position. the slide or carrier can also help locate the breechblock in its locking recess. The carrier or slide can be operated by lever or pump actions or by gas, for automatic and semiautomatic fire.

A tilting breechblock design is not confined to a tilt relative to the horizontal plane with the breechblock locking against the bottom part of the receiver (as described above). The FN Trombone uses a side locking design [23] and the breechblock of the Bren gun locks against the top part of the receiver. [24]

The M1895 Lee Navy is also of this type. The tilting action is achieved without a separate breechblock carrier but by the design of the cocking handle. When unlocking the breechblock, the cocking handle initially acts as a crank with a cam, which acts against the receiver to lift the rear of the bolt out of the locking recess. Further rearward pressure on the cocking handle then pulls the breachblock rearward. [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. A roller lock is commonly associated with firearms produced by Heckler & Koch. This type of breechblock can be adapted to cycle by lever, cocking handle, gas or recoil. The mechanism is usually designed so that a single action unlocks and then withdraws the breechblock using either a slide or levers.

The Henry rifle and subsequent Winchester rifles use an arrangement of levers referred to as a toggle, similar in operation to locking pliers. The breechblock is held closed when the two levers forming the toggle are in-line. [27]

Flapper locking (as used in the Degtyaryov machine gun) uses metal plates on either side of the breechblock that "flap out" like barbs on an arrow and lock into recesses in the receiver. The flaps can be retracted parallel to the block for unlocking. [28]

The straight-pull Heym SR 30 uses ball bearings for locking, similar to an air fitting connector. [29] The Blaser R93, another straight-pull action, is similar. It has a locking collet that consists claw-like segments instead of ball bearings. The claws have flat locking faces with a larger contact area to distribute load. These are arranged radially around the axis of the bolt. They extend to lock the bolt by engaging with an annular groove in the receiver and are retracted to unlock the bolt. [30]


Animation of simple blowback operation. Firearm simple blowback animation.gif
Animation of simple blowback operation.

Blowback actions use an in-line breechblock in which the breech is never locked and is held closed by spring tension alone. The force of the discharge is contained by a combination of spring force and the mass of the breechblock. They are used in semiautomatic and automatic firearms using low-powered cartridges. It is common in semiautomatic rifles and pistol chambered for .22 cal rimfire cartridges and many submachine guns. A variation is blow forward operation, in which the breechblock is fixed and the barrel moves. Delayed blowback uses additional mechanical devices retard or delay the rearward movement of the breechblock; however, the breechblock is not locked in place by such devices. It allows a lighter mechanism compared with simple blowback.

Floating actions

In most longarms, the barrel is firmly attached to the receiver and does not move relative to the receiver during operations. Most semiautomatic pistols firing the higher powered pistol cartridges use a locked-breech design. The action is manually cycled by moving the slide rearward. 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. When fired, recoil results in the same action. In many instances, the barrel and breechblock remain in-line. In the Browning Hi-Power and Colt's M1911 pistol, the barrel is tilted slightly to release it from interlocking ribs, so in this respect, it may be likened to a tilting breechblock, even though it is the barrel and not the breechblock that tilts.

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

Interrupted screw

Perhaps a variation on the rotating bolt, an interrupted screw provides greater strength than simple lugs while requiring only a partial rotation to release the breechblock. The Welin breechblock is such a design and is used on weapons with calibres from about 4 inches up to 16 inches or more.

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

Falling screwed breechblock

The Ferguson rifle used a tapered screw plug inserted perpendicular to the axis of the barrel. It was charged with ball and powder and required only one rotation to permit loading. While novel and effective, cost was a factor for its limited acceptance.

See also


  1. A less common type of breech is the split-breech design known as the "nutcracker". It is perhaps better described as a split chamber, since it is the chamber that contains the cartridge that is split. This type of breech consists of two counter-rotating sprockets, with a temporary chamber being formed where they touch. Relatively few guns have used this design. The prototype Fokker-Leimberger multiple-barreled machine gun used this design, but it had numerous problems with ruptured cases. [2] Another "Fokker Split Breech Rotary Machine Gun, ca. 1930" was donated to Kentucky Military Treasures; according to the museum record it "proved unsuccessful because of its inability to seal breech cylinders". [3] [4] A couple of 1920s US patents by other inventors also proposed to use this principle. [5] [6] The British also experimented with the design in the 1950s for aircraft guns, without success. [7] It has only been used successfully in low-pressure applications, such as the Mk 18 Mod 0 grenade launcher. [8]

Related Research Articles


Single-shot firearms are firearms that hold only a single round of ammunition, and must be reloaded manually after every shot. The history of firearms began with single-shot designs, then multi-barreled designs appeared, and eventually many centuries passed before multi-shot repeater designs became commonplace.

Bolt action Type of firearm mechanism

Bolt action is a type of manual firearm action that is operated by directly manipulating the bolt via a bolt handle, which is most commonly placed on the right-hand side of the weapon.

Action (firearms) Functional mechanism of breech-loading

In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.

A repeating rifle is a single-barreled rifle capable of repeated discharges between each ammunition reloads. This is typically achieved by having multiple cartridges stored in a magazine and then fed individually into the chamber by a reciprocating bolt, via either a manual or automatic action mechanism, while the act of chambering the round typically also recocks the hammer/striker for the following shot. In common usage, the term "repeating rifle" most often refers specifically to manual repeating rifles, as opposed to self-loading rifles, which use the recoil and/or blowback of the previous shot to cycle the action and load the next round, even though all self-loading firearms are technically a subcategory of repeating firearms.

Desert Eagle Semi-automatic pistol

The Desert Eagle or Deagle is a semi-automatic pistol known for chambering the .50 Action Express, the largest centerfire cartridge of any magazine-fed, self-loading pistol. Magnum Research Inc. (MRI) designed and developed the Desert Eagle. The design was refined and the pistols were manufactured by Israel Military Industries (IMI) until 1995, when MRI shifted the manufacturing contract to Saco Defense in Saco, Maine. In 1998, MRI moved manufacturing back to IMI, which later commercialized its small arms branch under the name Israel Weapon Industries. Since December 2009, the Desert Eagle Pistol has been produced in the United States at MRI's Pillager, Minnesota, facility. Kahr Arms acquired Magnum Research in 2010.

Stoner 63 Modular weapon system *Rifle *Carbine *Assault rifle *Light machine gun *Squad automatic weapon

The Stoner 63 is a 5.56×45mm NATO modular weapon system. Using a variety of modular components, it can be configured as a rifle, a carbine, a top-fed light machine gun, a belt-fed squad automatic weapon, or a vehicle mounted weapon. Also known as the M63, XM22, XM23, XM207 or the Mk 23 Mod 0 machine gun, it was designed by Eugene Stoner in the early 1960s. Cadillac Gage was the primary manufacturer of the Stoner 63 during its history. The Stoner 63 saw very limited combat use by United States forces during the Vietnam War. A few were also sold to law enforcement agencies.

Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge.

Mannlicher M1901 Semi-Automatic Pistol

The M1901 Mannlicher Self-Loading, Semi-Automatic Pistol was an early semi-automatic pistol design.

Rotating bolt Method of locking used in firearms

Rotating bolt is a method of locking the breech of a firearm closed for firing. Johann Nicolaus von Dreyse developed the first rotating bolt firearm, the "Dreyse needle gun", in 1836. Like the Mauser M 98 or M16, the Dreyse locked using the bolt handle rather than lugs on the bolt head. The first rotating bolt rifle with two lugs on the bolt head was the Lebel Model 1886 rifle. The concept has been implemented on most firearms chambered for high powered cartridges since the 20th century.

Bolt (firearms)

A bolt is the part of a repeating, breechloading firearm that blocks the rear opening (breech) of the barrel chamber while the propellant burns, and moves back and forward to facilitate loading/unloading of cartridges from the magazine. The firing pin and extractor are often integral parts of the bolt. The terms "breechblock" and "bolt" are often used interchangeably or without a clear distinction, though usually, a bolt is a type of breechblock that has a nominally circular cross-section.

Locked breech

Locked breech is the design of a breech-reloading firearm's action. This is important in understanding how a self-reloading firearm works. In the simplest terms, the locked breech is one way to slow down the opening of the breech of a self-reloading firearm when fired. The source of power for the movement is recoil.

Heckler & Koch HK21 General-purpose machine gun

The HK21 is a German 7.62 mm general-purpose machine gun, developed in 1961 by small arms manufacturer Heckler & Koch and based on the G3 battle rifle. The weapon is in use with the armed forces of several Asian, African and Latin American countries. It was also license-manufactured by Fábrica de Braço de Prata in Portugal as the m/968 and in Mexico by SEDENA as the MG21. In the German military (Bundeswehr) and the federal police (Bundespolizei) it is designated "G8".

Break action

Break action is a type of firearm action in which the barrel or barrels are hinged much like a door and rotate perpendicularly to the bore axis to expose the breech and allow loading and unloading of cartridges. A separate operation may be required for the cocking of a hammer to fire the new round. There are many types of break-action firearms; break actions are universal in double-barrelled shotguns, double rifles and combination guns, and are also common in single shot rifles, pistols, and shotguns, and can also be found in flare guns, grenade launchers, air guns and some older revolver designs. They are also known as hinge-action, break-open, break-barrel, break-top, or, on old revolvers, top-break actions.

M1895 Colt–Browning machine gun American machine gun

The Colt–Browning M1895, nicknamed "potato digger" because of its unusual operating mechanism, is an air-cooled, belt-fed, gas-operated machine gun that fires from a closed bolt with a cyclic rate of 450 rounds per minute. Based on a John Browning and Matthew S. Browning design dating to 1889, it was the first successful gas-operated machine gun to enter service.

SIG MG 710-3 General-purpose machine gun

The SIG MG 710-3 is a Swiss 7.62 mm general-purpose machine gun (GPMG) designed and manufactured by SIG - Schweizerische Industrie Gesellschaft. The weapon was developed as a commercial venture primarily for export, since the Swiss Army had already adopted the 7.5 mm MG 51 GPMG, produced by the federal arms factory W+F.

The following are terms related to firearms and ammunition topics.

Recoil operation is an operating mechanism used to implement locked-breech, autoloading firearms. Recoil operated firearms use the energy of recoil to cycle the action.

Tilting bolt action is a type of locking mechanism often used in self-loading firearms. Essentially, the design consists of a moving bolt driven by a piston with gas pressure from the gas port behind the muzzle. The bolt drops down into receiver recess and locks on bolt closing. Tilting the bolt up and down locks-unlocks in the breech. This tilting allows gas pressure in the barrel from firing the gun to lower to safe levels before the cartridge case is ejected. It can also be adapted for repeating rifles.

SIG Sauer System Type of action found in self-loading handguns

The SIG Sauer System is a type of action found in self-loading handguns. It is a refinement of designs based on the work of both John M. Browning and Charles Petter which began with the Browning Model 1910, progressed to the French Model 1935A, and later the SIG P210 handgun. This action first appeared in the United States on the Browning BDA .45 ACP caliber handgun around 1975. It represents a design which optimizes the cost of production of handguns while instilling high levels of accuracy and dependability. It is the basis for several SIG Sauer, Inc. designs which have been widely adopted for police, military, and civilian use and is the action used in the M17 and M18 sidearms of the United States Armed Forces. It has become a highly copied design found in many parts of the world today.

Repeating firearm

A repeating firearm, or repeater, is any firearm that is capable of being fired repeatedly before having to manually reload new ammunition into the weapon.


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