Gun automatics: delayed blowback action

In our previous article we discussed the simplest form of automatic gun action, known as “blowback”, or technically speaking, unlocked breech action. In this article we will discuss a slightly more complex form of the unlocked breech principle, known as “delayed” (or, in some sources, “retarded”) blowback. In essence, this system works the same way as simple blowback – it relies on back pressure, exerted by the powder gases onto the base of the cartridge case, pushing it backwards against the inertia of the breechblock and the force of the breechblock return spring. However, in this particular case some mechanical or physical means are employed to slow down (that is retard or delay) the initial opening of the breech block while the bullet is still traveling through the bore and the pressure inside the bore and case is high. There are many forms and implementations of delayed or retarded blowback systems, mostly developed before WW2 or shortly afterwards. Neither became very successful, and we will try to understand why.

The distinction between “delayed” and “retarded” blowback is basically that in “delayed” blowback systems the breechblock is momentarily stopped after the first several millimeters of rearward travel, to ensure that the thin walls of the cartridge are still supported by the chamber whilst the pressure is still high, and only a small part of the relatively thick cartridge base is exposed; once the bullet is out of the bore and pressure is down, the bolt is unlocked and able to cycle under the remaining pressure and/or inertia of the coupled mass (bolt carrier). In a “retarded” blowback system, the breechblock moves continuously from the very start, but its initial opening movement is slowed down by some additional means.
First delayed (retarded) blowback actions were patented in Europe before the turn of the 20th century. Some of the most notable, although unsuccessful early systems were patented by the famous Austrian gun designer Ferdinand von Mannlicher. One of his earliest self-loading designs was patented in 1893, and involved a military rifle using a “self-unlocking” rotary bolt which had angled bolt lugs entering spiral cuts inside the receiver. The general idea was to slow down the initial opening of the bolt via the friction between bolt lugs and their mortises in the receiver, and allow it to self-unlock under the pressure. However, the system worked too violently, resulting in many split cases. Later on, a similar design was used in the Thompson automatic rifle, designed in the USA, and (in simplified form) in the Villar-Perrosa machine pistol, designed in Italy. Only the latter worked more or less reliably, because it used relatively low-powered and short handgun ammunition. In fact, it probably would work equally well with simple blowback, provided with a somewhat heavier bolt.

von Mannlicher's patent for the self-opening rotary bolt delayed blowback action
Von Mannlicher’s patent for the self-opening rotary bolt delayed blowback action


Probably the most successful early design of a delayed (retarded) blowback weapon appeared in 1905-1907. It was a machine gun, designed by the German engineer Andreas Schwarzlose, and adopted by Austro-Hungarian, Swedish and some other European armies. Widely used during WW1, it remained in service until WW2 in Austria, Czechoslovakia, Hungary and the Netherlands. To ensure reliable extraction with a prematurely opening breech, Schwarzlose employed a built-in oiler that applies a small amount of lubricating oil to each cartridge during the chambering process. This helps to avoid cases sticking in the dirty or overheated chamber, which otherwise would result in stoppages or catastrophic gun failures due to torn case rims or separated cartridge heads. Retardation of the breechblock opening was ensured by the knee-shaped arrangement of two levers, supporting the breechblock. A similar solution was later used in Pedersen self-loading rifles, designed in the USA during the 1920s. John Pedersen also used a knee-toggle setup, although in a different arrangement than in Schwarzlose machine guns. He also applied lubricants, this time to each cartridge at the factory, using a wax-like coating. The Pedersen rifle operated well, but not well enough, especially when it was decided to switch from a relatively mild .276 cartridge (7x51mm) to the noticeably more powerful .30-06 (7.62x63mm) cartridge, and the Pedersen rifle eventually lost ground to the gas-operated rifle invented by John Garand.

Schwarzlose patent for his delayed blowback machine gun
Schwarzlose patent for his delayed blowback machine gun


The next production weapon that used delayed blowback action in which the breechblock actually stops during its recoil were the MKMO and MKPO submachine guns. These were designed in Switzerland by Kiraly and Ende and produced by SIG from 1934 until 1937. Those weapons were typical Swiss designs – extremely finely crafted, mechanically complicated and rather expensive. Not surprisingly, only a handful of these were made and sold (most notably to Vatican guards) before SIG engineers redesigned those as a simple blowback system in the subsequent MKMS / MKPS line of submachine guns. In the original MKMO / MKPO setup, its relatively small breechblock (bolt) had a projection at its rear top which engaged the rear edge of the top-located ejection port in the receiver, stopping the bolt after an initial 6 mm of recoil. The massive bolt carrier starts its recoil along with the breechblock and continues its rearward cycle while the breech block is stopped (delayed) by its projection. After some free recoil, the bolt carrier strikes the ramp surface at the bottom of the bolt and tips its rear end down, disengaging it from the receiver. Once the bolt is fully tipped down, the bolt / bolt carrier group continues its rearward movement under the inertia of the bolt carrier and the residual pressure retained in the bore. This sounds unnecessarily complicated for most pistol-type cartridges, and it actually was so, as proved by the aforementioned simple blowback MKPS system.

While SIG in Switzerland switched to a simple blowback system for its subsequent submachine guns, Pal Kiraly moved from Switzerland to Hungary to continue development of delayed blowback submachine guns. Here he eventually produced one of the more popular implementations of delayed (retarded) blowback systems, utilising a two part bolt with retarding lever. His system was originally patented as early as 1910 but only practically implemented in production weapons in 1939. This was in the 39M submachine gun, adopted by the Hungarian army. In this design a relatively light bolt is complemented by a heavier bolt carrier. A simple lever with two arms is attached to the bolt head through its pivot axis. When the bolt is closed, the lower arm of the lever rests against the stationary “shelf” in the receiver, and the upper arm bears against the front of the bolt carrier. Upon discharge, the rearward movement of the bolt head causes the lever to rotate and push back the bolt carrier with greater velocity, thus momentarily increasing its inertia force exerted against the pressure of the powder gases. Once the initial “retarded” movement of the bolt head is complete, the lower arm of the lever leaves its seat in the receiver and from this moment on all parts of the bolt group recoil as one, under the residual pressure of the bore and inertia of the moving parts. This system worked more or less well with handgun cartridges, but in fact it was redundant due to the relatively low pressures involved with submachine guns and their ammunition. This same system was later more or less successfully carried forward by French engineers, first in their 7.5x55mm AA M52 universal machine gun, and then in the 5.56x45mm FA MAS assault rifle. In either case, this system worked “reasonably well”, but it proved to be too sensitive to headspace issues and to the quality of production and materials used in cartridge cases (especially in FA MAS). As a bit of historical trivia we can also note that during the 1950s and 1960s Soviet designers developed a number of experimental assault rifles and light machine guns utilising a Kiraly-type lever-retarded system. These systems proved to be noticeably simpler and cheaper than the Kalashnikov gas operated action, and also registered less dispersion while firing in full automatic. However, all these systems were found to be inherently less reliable than Kalashnikov’s, especially under harsh environmental conditions, and all were dropped out of the competition.

Kiraly patent for his lever delayed blowback system
Kiraly patent for his lever delayed blowback system


Despite, let us say, the marginal popularity and success of delayed and retarded action systems during the first half of the 20th century, one particular company became a strong proponent of this system during the second half of the same era. Here we speak about the famous German company that became commercially successful especially due to their delayed (retarded) blowback weapons of many types. As you can rightfully guess, this company is called Heckler und Koch and we are speaking about the roller-delayed system. The origins of this system can be traced back to two parallel German developments dated to the 1944-45 era. One was the experimental MG.42(V) / MG.45 universal machine gun by Grossfuss, and another the great 06(H) / Stg.45(M) assault rifle by Mauser. Both systems started as roller-locked actions based on the highly successful MG.42 machine gun. In both cases it was found that with careful redesign, rigid locking can be replaced with a self-opening system in which rollers act as a means of retarding the bolt head, at the same time accelerating the heavier bolt body, by riding inclined surfaces in the barrel extension. This system was refined during the 1950s by German engineers who emigrated to Spain after the end of WW2. There it was first successfully used in the CETME Mod.B automatic rifle, which was later licensed to H&K in Germany and in modified form, adopted by the German army as the 7.62×51 G3 rifle in 1959. Inherent extraction issues typical to delayed / retarded blowback systems were solved here by using a so-called “fluted chamber” that allowed some of the powder gases from the bore to flow rearward through the flutes and thus provide some compensating pressure from outside of the cartridge case, decreasing the friction during the initial, high pressure stages of blowback.

Starting with this rifle, HK engineers then developed a broad family of small arms, all utilising this same roller-delayed (retarded) system. This family initially included the G3 as the parent weapon, plus HK 11 light machine gun and HK 21 universal machine gun (all chambered for 7.62×51 NATO ammunition), as well as the smaller HK P9s pistol (in 9×19 and .45) and one of the most successful submachine guns of the post-WW2 era, the HK MP5 (in 9×19, and, later on, in .40SW and 10mm Auto). The second generation of roller delayed systems included 5.56mm weapons such as the HK 33 and G41 rifles, HK 13 and HK 23 light machine guns, as well as the HK 7.62mm PSG-1 sniper rifle. The G3 rifle met with significant international success, being adopted by at least 30 militaries worldwide and produced under license in several countries (including Turkey, Pakistan and Iran). The MP5 became an international hit, being adopted by God only knows how many militaries and law enforcement organizations around the world. Incidentally, today HK MP5 remains the only mass-produced delayed (retarded) blowback weapon, despite the fact that similar results with 9×19 ammunition can be easily obtained with simpler and cheaper “simple” blowback weapons. Similar roller-delayed actions were also used in a small number of other weapons, such as the US-made Calico submachine guns or more recent SRM 1216 semi-automatic shotguns (also of US production). However, no other weapon with a similar action could compete with the MP5 in terms of popularity and success.

Vorgrimmler's patent for roller-delayed blowback system
Vorgrimmler’s patent for a roller-delayed blowback system


Notably less successful was another German design of a type of retarded blowback action, which used the pressure of powder gases to slow down the initial opening of the breech block. First used in 1945 in the experimental VG.1-5 semi-automatic carbine by Rheinmetall, it was later employed in the Austrian made Steyr GB-18 pistol and in the German made HK P7 pistol (in modified form). In the case of the Rheinmetall rifle and Steyr pistol, powder gases were fed from the bore (via dual ports) into the sliding cylinder fitted around the barrel; there they expanded, acting against the annular forward end of the cylinder, which in turn was connected to the breechblock. In the HK P7 pistol, the gas cylinder was located below the barrel, but in all cases the gas-retarded system was prone to rapid overheating and the clogging of gas powder residue.

patent diagram for Steyr GB gas delayed blowback system
Patent diagram for Steyr GB gas delayed blowback system


Overall, we must conclude that delayed / retarded blowback systems never found great popularity. For low-powered pistol-caliber weapons its advantages are often too marginal to warrant the additional expenditures necessary compared to simple blowback weapons. In more powerful weapons firing intermediate or full power rifle ammunition, the relative simplicity of the unlocked breech system is offset by decreased reliability and inherent sensitivity to extraction issues and cartridge case materials. One good example of the latter issue is modern composite (polymer / metal) case ammunition, which works reasonably well in locked breech designs but causes dismal results in delayed blowback actions with fluted chambers.