Delayed blowback is a form of blowback operation intended for higher pressure rounds such as rifle rounds. It uses an operation that delays the opening of the bolt until the gas pressure drop to a safe level to reload when firing. Becouse of the higher pressure, rifle calibre delayed blowback firearms usually have fluted chambers to ease extraction, notably the FAMAS and Heckler & Koch G3.


Lever delayedEdit

FAMAS Bolt-gr

A schematic of the lever-delayed blowback mechanism used in the FAMAS assault rifle.

Lever-delayed blowback utilizes leverage to delay the opening of the breech.[1] When the cartridge pushes against the bolt face, the lever moves the bolt carrier rearward at an accelerated rate relative to the light bolt. This leverage significantly increases resistance and slows the movement of the lightweight bolt. John Pedersen patented the first known design for a lever-delay system.[2] The mechanism was adapted by Hungarian arms designer Pál Király (a.k.a. Paul de Kiraly) in the 1930s and first used in the Danuvia 43M submachine gun. Other weapons to use this system are the TKB-517/2B-A-40 assault rifles, the AVB-7.62 battle rifle, the San Cristobal Carbine, the FAMAS,[3] the BSM/9 M1 and FNAB-43 submachine guns, the Hogue Avenger and Benelli B76 pistols, the Sterling 7.62 and AA-52 machine guns.

Roller delayedEdit

Roller-delayed blowback was first used in the experimental MG 42 derivative MG 42V and the 1945 Mauser StG 45(M) prototypes. Roller-delayed blowback operation differs from roller-locked recoil as seen in the MG 42. Unlike the MG 42, in roller-delayed blowback the barrel is fixed and does not recoil. As the bolt head is driven rearward, rollers on the sides of the bolt are driven inward against a tapered bolt carrier extension. This forces the bolt carrier rearward at a much greater velocity and delays movement of the bolt head. The primary advantage of roller-delayed blowback is the simplicity of the design compared to gas or recoil operation.[4]

After WWII, former Mauser technicians Ludwig Vorgrimler and Theodor Löffler perfected this mechanism between 1946 and 1950 while working for the French Centre d'Etudes et d'Armament de Mulhouse (CEAM). The first full-scale production rifle to utilize roller-delay was the Spanish CETME followed by the Swiss Sturmgewehr 57, and the Heckler & Koch G3 rifle. The HK MP5 is the most common weapon in service worldwide still using this system. The P9 pistol also uses roller-delayed blowback. The Czech URZ AP machine gun is roller locked.

Gas delayedEdit

Gas-delayed blowback should not be confused with gas operation. The Bolt is never locked, and so is pushed rearward by the expanding propellant gases as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt. It was used by some World War II German designs for the 7.92×33mm Kurz cartridge, including the Volkssturmgewehr 1-5 rifle (with little effectiveness) and the Grossfuss Sturmgewehr (with slightly more efficiency),[5] and after the war by the Heckler & Koch P7, Steyr GB and M-77B pistols.

Chamber-ring delayedEdit

When a cartridge is fired, the case expands to seal the sides of the chamber. This seal prevents high-pressure gas from escaping into the action of the gun. Because a conventional chamber is slightly oversized, an unfired cartridge will enter freely. In a chamber-ring delayed firearm, the chamber is conventional in every respect except for a raised portion at the rear of smaller diameter than the front of the chamber. When the case expands in the front of the chamber and pushes rearward on the slide, it is slowed as this raised portion constricts the expanded portion of the case as the case is extracted. The Seecamp pistol operates on this principle.

Off-axis bolt travelEdit

John Browning developed this simple method whereby the axis of bolt movement was not in line with that of the bore.[6] The result was that a small rearward movement of the bolt in relation to the bore axis required a greater movement along the axis of bolt movement, essentially magnifying the resistance of the bolt without increasing its mass. The French MAS-38 submachine gun of 1938 utilizes a bolt whose path of recoil is at an angle to the barrel. The Jatimatic and TDI Vector use modified versions of this concept.

Toggle delayedEdit

In toggle-delayed blowback firearms, the rearward motion of the breechblock must overcome significant mechanical leverage.[7][8][9] The bolt is hinged in the middle, stationary at the rear end and nearly straight at rest. As the breech moves back under blowback power, the hinge joint moves upward. The leverage disadvantage keeps the breech from opening until the bullet has left the barrel and pressures have dropped to a safe level. This mechanism was used on the Pedersen rifle and Schwarzlose MG M.07/12 machine gun.[10][8]

According to an United States Army Materiel Command engineering course from 1970, "The retarded blowback type, because it depends on a linkage system to control bolt recoil that is extremely sensitive to geometric proportions, does not have the reliability of the [lever-]delayed type either in theory during design, or in practice during development and usage. The large load applied to the linkage while in motion adversely affects the gun's durability."[11]

Hesitation lockedEdit

John Pedersen's patented system uses a separate breech block within the slide or bolt carrier. When in battery, the breech block rests slightly forward of the locking shoulder in the frame. When the cartridge is fired, the bolt and slide move together a short distance rearward powered by the energy of the cartridge as in a standard blowback system. When the breech block contacts the locking shoulder, it stops, locking the breech in place. The slide continues rearward with the momentum it acquired in the initial phase. This allows chamber pressure to drop to safe levels while the breech is locked and the cartridge slightly extracted. Once the bullet leaves the barrel and pressure drops, the continuing motion of the slide lifts the breech block from its locking recess through a cam arrangement, continuing the firing cycle. The Remington 51 pistol and SIG MKMS submachine gun were the only production firearms to utilize this type of operating system.



Radial delayed blowback operation uses the rotation of the bolt head to accelerate the bolt carrier. The bolt locking lugs use angles that rotate the bolt as it travels rearward under conventional blowback. As the bolt rotates, it must accelerate the bolt carrier to the rear through an adapted cam-pin slot. This accelerates the effective mass of the bolt carrier, slowing the speed of the bolt head. The delay allows gas pressure to drop to a safe level prior to extraction without the penalty of a heavier bolt carrier assembly. Radial delayed blowback is similar to roller and lever-delayed blowback in that it uses the mass of the bolt carrier moving at a faster rate than the bolt head to delay the action from opening. The CMMG Mk45 Guard uses this operation.



First used on the Mannlicher Model 1893 rifle, the bolt in screw-delayed blowback was delayed by angled interrupted threads which required a quarter twist to unlock. John T. Thompson designed an autorifle that operated on a similar principle around 1920 and submitted it for trials with the US Army. This rifle, submitted multiple times, competed unsuccessfully against the Pedersen rifle and Garand Model 1919 primer-actuated rifle in early testing to replace the M1903 Springfield rifle.[12] This operation is one of the most simple forms of delayed blowback but unless the ammunition is lubricated or uses a fluted chamber, the recoil can be volatile especially when using full length rifle rounds as well as rotation of the bolt should be at least 90° to prevent ruptured cartridges. Mikhail Kalashnikov later developed a prototype submachine gun in 1942 that operated by another form of screw-delayed blowback principle,[13] which is also found on the Fox Wasp carbine. A pair of telescoping screws delayed rearward movement of the operating parts during the firing cycle. This weapon was ultimately not selected for production.[14]

Other blowback systemsEdit


Floating chamberEdit

David Marshall Williams (a noted designer for the U.S. Ordnance Office and later Winchester) developed a mechanism to allow firearms designed for full-sized cartridges to fire .22 caliber rimfire ammunition reliably. His system used a small 'piston' that incorporates the chamber. When the cartridge is fired, the front of the piston is thrust back with the cartridge giving a significant push to the bolt. Often described as accelerated blowback, this amplifies the otherwise anemic recoil energy of the .22 rimfire cartridge.[15] Williams designed a training version of the Browning machine gun and the Colt Service Ace .22 long rifle version of the M1911 using his system. The floating chamber is both a blowback and gas operated mechanism.[16]

Primer actuatedEdit


Primer actuated blowback.

Primer actuated firearms utilize blowback force to set the primer back to operate a mechanism to unlock and cycle the firearm. John T. Kewish[17] and John Garand were the first to develop the system in an unsuccessful bid to replace the M1903 bolt action rifle in the early 1920s.[18] The Soviet designer Fedor Tokarev also experimented with this principle in rifles in the 1930s.[19]

The U.S. military adopted ammunition with crimped primers that do not set back. Another Garand design was eventually accepted. A light rifle chambered in .30 carbine was also trialed. AAI Corporation used their developmental piston primer mechanism in a rifle submitted for the SPIW competition.[20] During the Abakan trials, the Postnikov APT used primer unlocking blowback. A similar system is used in the spotting rifles on the LAW 80 and Shoulder-launched Multipurpose Assault Weapon use a 9mm, .308 Winchester based cartridge with a .22 Hornet blank cartridge in place of the primer. Upon firing, the Hornet case sets back a short distance, unlocking the action.[21]

Case setbackEdit

The case cartridge itself has been used experimentally to actuate the action similar to Garand's primer-actuation. Known prototypes using this method of operation include two 1936 rifle designs, one by Mihail Mamontov and another by Makar Goryainov at TsKB-14, and a 1980's design by A.F. Baryshev. The Mamontov and Goryainov rifles are only partially automatic; only the bolt unlocking is powered by the gasses pushing the cartridge back, while the rest of the cycle (ejection, reloading) is done manually as in a traditional bolt-action rifle. A major problem with using the case cartridge as piston is that its motion is much faster (about 1 ms) compared compared to tapping gas further down the bore through a piston—about 5 ms in the Dragunov sniper rifle, which used the same cartridge as Mamontov's rifle. Baryshev made a fully automatic, but rather bulky mechanism that used a mechanical delay. In his system, the case cartridge pushed back a tilting bolt face, that upon reaching a certain angle pushes backwards an unlocking lever that continues farther before unlocking the bolt. The GRAU however still gave a negative evaluation of Baryshev's gun, pointing out that the main problems with reliability of firearms using the cartridge case a piston were known since the 1930s and still unsolved.[22]

Limited-utility designsEdit

Blish lockEdit

Main article: Blish lock

The Blish Lock is a breech locking mechanism designed by John Bell Blish based upon his observation that under extreme pressures, certain dissimilar metals will resist movement with a force greater than normal friction laws would predict. In modern engineering terminology, it is called static friction, or stiction. Lubrication or fouling would completely defeat any advantages. His locking mechanism was used in the Thompson submachine gun, Autorifle and Autocarbine designs. This dubious principle was later eliminated as redundant in the .45 caliber submachine gun. Any actual advantage could also be attained by adding a mere ounce of mass to the bolt.

Savage rotating barrelEdit

The Savage system employed the theory that the rifling in the barrel caused a rotational force that would hold the gun locked until the projectile left the barrel. It was later discovered that the bullet had left the barrel long before any locking could occur. Savage pistols were in fact operating as pure blow back firearms.[23]


  2. Template:US Patent
  3. Modern firearms - GIAT FAMAS assault rifle (France) article
  4. Stevens, R. Blake, Full Circle: A Treatise on Roller Locking, Collector Grade Publications (2006). ISBN 0-88935-400-6.
  5. Юрий Пономарёв "Автомат Хорна", КАЛАШНИКОВ. ОРУЖИЕ, БОЕПРИПАСЫ, СНАРЯЖЕНИЕ 2006/9, pp. 20-26
  6. Template:US Patent
  7. Cliff Carlisle, Japanese Pedersen Semi Auto Rifles & Carbines, article
  8. 8.0 8.1 Template:Cite book
  9. Automatic Weapons, AMC pamphlet no. 706-260, February 1970, page 2-40
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  20. Flirting With Flechettes: The US Army's Search for the Ideal Rifle Projectile May 2000 article
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  23. Hatcher, Julian, Hatchers Notebook, The Military Service Press Company (1947), pp. 259-261. ISBN 0-8117-0795-4.