Breech Forces are a series of physical and mechanical forces that act on the breech of a firearm. At first a regular firearm has to withstand the breech forces so that hot gases can not exit the rear and endanger the weapon or the shooter. At second these forces can be used to operate the action of an automatic firearm.
There are six known breech forces: Primer Pushout, Case Exposition, Bullet Extraction Impulse, Blowback, Bullet Recoil and Gas Recoil (Rocketeffect).
Primer Pushout[]
Primer pushout: The primer acts as pistol (red), the cade acts as cylinder (blue)
The moment the firingpin hits the primer the propellant will transform into propellant gas this gas takes up more space, then the former propellant. This creates an enormous pressure inside the cartridge case. This pressure works equally to all insides of the case. The first point the gas will do work on is the primer. The gas will try to push the primer out of its pocket. The primer acting as a piston, the primer pocket acting as its cylinder. Most conventional firearms will prevent this movement by supporting the primer with the breech face and especially military amunition got their primers fixed in its pocket, to prevent movement.
But some firearms use this fore to actuate their action, these guns are called Primer actuated. Some experimental guns like an M1 Garand Prototype and Spotting rifles use this system.
Case Expansion[]
Case extention: The two piece case acts as piston (red) and cylinder (blue)
The gases inside the casing will try to expand the rather soft case material to all sides. In an conventional firearm this expansion is prevented by the chamber and proper headspacing, do prevent case rupture do to overstretching of the case material. But some rare firearms are using the force of an expanding case such as the UARC autorifle prototype. There are two different kinds of expanding cases. Special design expanding cases mostly two piece constructions acting like telescopes and normal cases that expand by stretching the case material. Later construction are not proven yet and it possible that firearms using case stretching are just operation by the Blowback principle.
Firearms that use this method to operate are often called Headspace actuated.
Bullet Extraction Impulse[]
Bullet extraction impulse: The bullets action(yellow) is received by the case(green) as a reaction, the gases are the transmitter (pink)
The gases inside the case will push out the bullet. Because the bullet represents a moving mass the third Newtons law comes into play creating an counter impulse. This impulse will use the gasses as a fluid mechanical transmitter to act on the case. Against the common believe this is not recoil, because the case is lighter then the bullet. In most firearms the bullet is to light and the bullet extraction distance is to short for the bullet extraction impulse to be strong enough to move the breech of a blowback operated firearm.
But there is one firearm known to use this extraction impulse, the Russian PSS Pistol. Its heavy bullet together with its long extraction distance make this pistol an bullet extraction operated firearm.
Blowback[]
The high pressure gases inside the case will push the bullet out of the case mouth into the barrels throat. Before that the gases are trapped inside the closed system and can only act indirectly on the breach. This chances if one of two thing will happen.
Blowback with early throat contact: The case is acting as cylinder(red) and the bullet as piston(red)
First, if the bullet didn't got any free bore and is ably to make contact to the barrels throat, before fully exiting the cartridges mouth, the bullet will be provided a contact point to push the casing backward against the breech face. The bullet will act as piston and the case will act as a cylinder, that is pushed against the breech of the gun. This will only happen in guns that have little to no free bore.
Blowback with freebore: The case is acting as a piston(red), the barrel is acting as the cylinder(blue)
Second, if the bullet has left the case mouth and is entering the free bore zone the case will no longer be an enclosed system. Therefore the gas is able to push back on the cartridge case. The case is acting like a hollow piston pushing against the breech face. If the breech is not locked, the gases are able to push the bolt backwards. If the bolt is sufficiently locked these gases can not act on the breech ore move the gun, because their ability is limited in an local frame of reference.
Guns using the blowback force are called blowback operated. They are gas operated but often make their own category for ease of understanding. Guns that use this system are PM Makarov, F1 submachine gun, Uzi. Delayed blowback examples are H&K G3, FAMAS etc.
Bullet Recoil[]
Bullet Recoil: The bullets action(yellow) is received by breech and barrel(green) as a reaction, the gases are the transmitter (pink)
As soon as extraction resistance and throat resistance have been overcome by he bullet and the majority of the propellent powder is burning, the bullet will gain a lot of speed. According to Newton's third law this movement will result in an contra impulse. This impulse will use the gases as an fluid mechanical device to act on the breech face. Because of it being an impulse the bullet recoil can act in a global frame of reference. In opposite to blowback the bullet recoil is able to move breech faces that have been locket to a barrel ore the entire gun, causing the kick.
Weapon that utilize the bullet recoil are called recoil operated. They are impulse operated, make their own category and are easy distinguishable by the fact that they won't cycle with blank rounds. Well known guns are M1911 pistols, Beretta M9, Browning Auto-5, Chauchat.
Gas Recoil[]
Gas recoil:The exiting gases action(yellow) is received by breech and barrel(green) as a reaction, the gases in the barrel are the transmitter (pink)
When the bullet leaves the barrel the high pressure gases will leave the barrel with high speed. This rapid movement of the gases particles will result in another reaction because of Newton's third law. The created contra impulse will again use the gas as a fluid mechanical device to transmit this impulse to the breech face. As well as bullet recoil, gas recoil will act in a global frame of reference and is able to push breeches to the rear that have been locked to a barrel ore move the entire gun backwards, causing more kick. Gas Recoil is also known as the Rocket effect.
There are know known guns using only gas recoil to operate but it helps some recoil operated machine guns to cycle. These are usually equipped with barrel fixed Muzzleboosters like on the German MG 08.
Overview[]
| Breech Force | Type | Force | Frame of reference |
| Primer Pushout | Gas pressure | weak | local |
| Case Expansion | Gas pressure | strong | local |
| Bullet Extraction Impulse | Impulse | weak | global |
| Blowback | Gas pressure | strong | local |
| Bullet Recoil | Impulse | strong | global |
| Gas Recoil | Impulse | medium | global |
Sources[]
- Sub-Machine Gun, The Development of Sub-Machine Guns and their Ammunition by Maxim Popenker & Anthony G. Williams, 2011
- Engineering Design Handbook - Gun Series - Automatic Weapons by United States Army Materiel Command, 1970
- The Machine Gun Vol. IV Design Analysis of Automatic Firing Mechanism by Georg M. Chinn, 1955
- Hatcher's Notebook, A Standard Reference Book by Julian S. Hatcher, 1948