Self-loading firearms can use several different types of operating systems.

 

(1) Gas operated -- probably the most common.

(2) Recoil operated -- not quite as common.

(3) Delayed blowback (roller/flap locked breech).

(4) Blowback -- suitable for low-power cartridges only.

 

Blowback firearms are "locked" to contain the pressures of the cartridge through a combination of spring force and weight of the operating parts (slide or bolt).  Since there is no mechanical locking device, calibers are limited to .22 caliber throu .45 APC (pistol calibers).

 

Delayed blowback (or roller/flapped locked breech) firearms have operating mechanisms quite different from items (1), (2), and (4).  Examples of this system are the Spanish CETME/German HK91/G3-series rifles, HK MP5 submachineguns, French AAT-52 General Purpose Machine Gun.  Delayed blowback arms use either locking rollers (that look like roller bearings) or a pair of locking flaps that hold the bolt locked to the barrel extension or breech to contain the high pressure of rifle cartridges.  However, the delayed blowback firearm has no primary extraction to loosen the fired case in the barrel's chamber.  There are two ways to make sure the bolt does not unlock before breech pressures drop to safe levels: (1) lubricate the cartridge case or (2) flute the chamber walls of the barrel to "float" the fired case on burning propellant gases.  Lubrication of cartridge cases has obvious disadvantages of collecting dirt and grit that can jam the firearm.  Therefore, fluting the barrel chamber is preferred.  Cases fired in fluted chambers exhibit black, parallel marks around the end of the fired case from the burned propellant.  Extraction and ejection of fired cases is very violent with these arms.  Cases are thrown for a long distance and sometimes have a crease in the side where they hit the side of the ejection port.  CETME/HK91/G3-series rifles should be fired with 7.62x51 NATO ammo because the brass is harder than commercial .308 Winchester.  Use of the latter ammunition can cause problems if used with these rifles.

 

Recoil operation means that the barrel of the firearm has to move backwards (under the power of the muzzle blast) to unlock the breech of the firearm.  Examples of such arms are the Browning Auto 5 (or A-5) shotgun, M1911-series pistol, M1941 Johnson rifle, and M1919A4 Browning light machine gun.  Some guns like the M1919A4 use a muzzle booster (or cap) that goes over the end of the barrel to trap more of the muzzle blast and move the barrel back faster.  Recoil operation was the first of the self-loading operating systems and is very efficient, but has been eclipsed by the more common gas operation system.  It should be noted that the WW2 German machine guns MG34 and MG42 both used recoil operation, but incorporated the lflap (MG34) or roller (MG42) method of locking the bolt and barrel together to seal the breech.

 

Gas operation uses a gas port or hole drilled in the barrel to unlock the breech of the firearm.  Part of the burning propellant gas is tapped off and directed against a gas piston, or tappet, or through a gas tube to unlock the bolt.  Examples of such arms are the M1 Garand rifle (gas piston or "operating rod"), Russian Simonev SKS-45 carbine (gas piston and tappet), and AR-15/M16-series rifles (gas tube and bolt/bolt carrier with gas rings).

 

The very first self-loading design was the Maxim fully automatic machine gun.  The Maxim was recoil operated and could shoot an unheard of 666 rounds per minute --  feat previously unimagined in 1888.  It was not until the early 1920's that a reliable self-loading semi-automatic rifle was developed.  There were attempts from the turn of the 20th century, but results were overly complicated, too heavy, or both.  The M1 Garand rifle was the first mass-produced self-loading military rifle issued to troops in great numbers (1937).  American troops had a significant firepower advantage over their allies and opponents who were almost entirely armed with bolt action repeaters.