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Working systems: blowback-operated firearms

Russian expert Maxim Popenker discusses working systems, starting with blowback operation
The use of straight blowback operation is nowadays mostly limited to pistol-caliber carbines and sub-machine guns
Russian expert Maxim Popenker discusses working systems, starting with blowback operation
A patent diagram dating before World War II, demonstratring the classic blowback-operated SMG action derived from the Bergmann Mp18
Russian expert Maxim Popenker discusses working systems, starting with blowback operation
A post-WW2 patent demonstrating an upgraded version of the blowback operating principles for sub-machineguns: models such as the CZ Samopal Vz.25, the UZI or the Beretta PM-12 have since concentrated most of the bolt's weight in front of the breech face

Despite its obvious advantages in terms of simplicity and low cost, the blowback operation also sports several noticeable drawbacks.

First and foremost, it is somewhat sensitive to the quality of cartridge case material and to the smoothness of the barrel chamber to ensure reliable operation and avoid case ruptures or torn rims.

Second, the mass of the breechblock has to balance the strength of the spring and the length of the breechblock cycle: if spring is too weak or the bolt is too light, the resulting force will be insufficient to keep the cartridge inside the chamber while pressure within the system is high. This could cause potentially disastrous conditions of ruptured or separated cases. If the spring is too strong, the gun will be hard to cycle manually; if the bolt is too heavy, its movement will cause excessive vibrations when fired, disrupting accuracy.


Also, as the cartridge power grows (in this case, the “power” is given by the internal pressure level multiplied by the inner area of the cartridge base), so grows the bolt weight level necessary to keep the cartridge in the chamber. Normally, sub-machine guns chambered to fire the 9x19mm Luger ammunition sport a breechblock weighting around a 0.5 kg.

With an intermediate cartridge – such as the Russian 7,62x39mm M43 – the necessary weight of the breechblock would rise to about 2 kilograms, and with earlier typical military rifle cartridges, e.g. 7,62x51mm NATO, the breechblock would have to weight something around four to five kilograms, making a blowback system absolutely impractical (not mentioning the inherent unreliability due to the very high pressure levels involved).

This ain't a great deal when dealing grenade launchers, since those systems generate noticeably lower pressures and are usually mounted on tripod or vehicles for stability; the increased dispersion, caused by a heavy breechblock oscillating back and forth inside the gun is also not an issue for weapons which are normally used against “area targets” rather than against “point targets”.

Russian expert Maxim Popenker discusses working systems, starting with blowback operation
In the early 1970s, the German-based Heckler & Koch was technically successful – commercially, not so much – in creating the VP70, a very simple blowback pistol that would fire the powerful 9x19 ammunition
Russian expert Maxim Popenker discusses working systems, starting with blowback operation
A Hi-Point pistol, manufactured in the United States and based on a simple blowback operation, is extremely cheap to produce and sports a low retail price – but it also comes with a necessarily heavy and very bulky slide in order to be sturdy enough to fire the powerful .40 Smith & Wesson round

As mentioned above, the usefulness of the blowback operation principle is limited by the rearwards pressure level, generated by a cartridge being fired, which dictates the weight of the breechblock and the strength of the spring.


For handguns, which are normally limited in weight to 1 kg and even less, the 9mm Luger caliber represents the cut-off line for the use of a straight blowback system. While some practically working pistols were indeed built to fire the 9x19mm round using a blowback system – such as the Heckler & Koch VP70, or the currently-produced, US-made Hi-Point line – most blowback-operated handguns are chambered to fire relatively low-power rounds such as the 9x18mm Makarov round, the .380 ACP caliber, and others even smaller.


Shoulder-fired guns such as pistol-caliber carbines and sub-machine guns can accommodate heavier bolts, and are thus often chambered in most major pistol calibers such as 9x19mm, .40 Smith & Wesson and .45 ACP, while retaining a straight blowback operation. No blowback-operated rifle or machine-gun chambered to fire an intermediate or full-power cartridge was ever mass produced.

Russian expert Maxim Popenker discusses working systems, starting with blowback operation
Want to know more about what's in the heart of your favorite firearm? Stay tuned on these pages for the next articles of the series!
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