Gun automatics: Blowback Action

Simple Blowback Operated, Automatic Action:

Generally speaking, firearms can be separated into two separate classes – manually operated or automatic. Manually operated guns require the deliberate actions of the operator to extract a fired cartridge case, load a fresh cartridge and cock the firing mechanism for the next shot. Automatic weapons do the same without any effort from the operator, that is to say.. automatically. Automatic weapons can be further separated into several classifications and categories.

The first classification is distinguished by the source of energy that cycles the action. There is a very large subclass of self-loading guns that use the energy of the previous shot to cycle for the next, and then a relatively small subclass of externally powered guns that use an electric or hydraulic motor to cycle the action. In the subsequent series of articles we will discuss the various types of actions used by automatic firearms. Firstly, weapons utilising the simplest form of automatic action use gas pressure alone, from each discharge of the gun to cycle it – that is:

Blowback Action.

It must be noted that most modern cartridges for firearms (except for some rare, very specific ‘captive piston’ silenced designs) are not strong enough to contain the pressures generated by the exploding powder. Thus, the cartridge has to be supported from the sides by the chamber area of the barrel and from the rear, by a device called the breechblock. Typical pressures generated by a centerfire cartridge are in the range of 2,000 kg/cm2 and up. Thus, the forces exerted onto the cartridge from the inside can be measured in the range of tons.

Thankfully, the time in which this force is actually being applied is extremely short. Most early manually operated guns used some form of rigid connection between the breechblock and the barrel at the moment of firing, such as a ‘locking’ bolt.

Automating these systems was not an easy task. However, the simple physics of inertia came to the designers’ aid. It was found that by using an unlocked, horizontally sliding breechblock, allowed to move freely and supported from the rear by a strong spring, there would be enough inertial mass (weight) to hold the cartridge inside the chamber while the bullet travels down the bore and the pressure inside the system is still extremely high. Of course, in the case of blowback operation, the cartridge and breechblock are freely moving backwards as well, right from the moment of ignition. However with a properly designed blowback system this initial movement is too tiny to expose the thin cartridge walls before the pressure has had time to drop to a safe, much lower level.

Another benefit of this kind of system is that the initial movement and residual pressure gives the empty case enough momentum to be ejected from the chamber and out of the gun, thus already automating the first part of the cycle. The heavy breechblock, acting on its own inertia, compresses the spring until it bottoms out. Once the breechblock is furthermost to the rear, its forced forwards again by the stored energy of the coiled spring. It loads a fresh cartridge as it goes by the magazine opening and this finishes the firing cycle, ready to fire again.

Depending on the system of ignition, this can also cock the separate hammer or striker; alternatively, the breechblock itself can contain a firing pin to fire the next round upon full closure. This is a very simple and effective system, and generally describes blowback action.

Its earliest applications were found in the semi-automatic pistols of the late 19th Century. The cartridges at the time were well suited for such a system, as they had relatively short cases and low pressure powder charges, so as not to overpower the system. To name a few of these examples:

  • A number of Bergmann pistols starting with the 1894 model. Some of the earliest models didn’t even have an ejector, and their grooveless cases were extracted and ejected using simple residual pressure in the chamber.
  • An experimental pistol by Hiram Maxim and his assistant Louis Silverman – the ‘Maxim Silverman model 1896’ of which only 3 were ever made. (For more on this gun see here).
  • One of the most famous gun designs of the period, Browning’s FN M1900 and FN M1903 pistols, which set the stage for most of the semi-automatic pistols to come.

 

Patent diagram for one of the earliest Bergmann blowback pistols
Patent diagram for one of the earliest Bergmann blowback pistols

Bergmann model 1896 pistol
Bergmann model 1896 pistol that used blowback action

John Browning's patent for his first sucessful semi-automatic pistol, model FN 1900
John Browning’s patent for his first successful semi-automatic pistol, model FN 1900

FN Browning model 1900 pistol
FN Browning model 1900 pistol, the first really successful and mass-produced blowback semi-automatic pistol

 

From 1918, this system was successfully applied to a new class of fully automated weapons, the submachine gun. The classic design of its class and the first, the Bergmann MP.18, used a very simple cylindrical breechblock that cycled back and forth inside a tubular receiver. It was a highly successful setup, subsequently copied throughout the world, and is still used in a number of submachine guns in service with various military and police forces, such as the UZI or the Skorpion vz.61.

Beginning in WW2, a number of experimental and, later, production submachine guns adopted a ‘wrap-around’ bolt system, inspired by semi-automatic pistols, where a noticeable amount of the bolt mass is concentrated just in from of the bolt face, above or around the breech portion of the barrel. This helps to reduce the overall length of the gun while maintaining the mass necessary for the operation of the breechblock.

Bergmann MP.18 submachine gun
Bergmann MP.18 submachine gun

 

Diagram of the early blowback-operated submachine gun with bolt located behind the barrel
Diagram of the early blowback-operated submachine gun with bolt located behind the barrel

later design of the submachine gun, with most of the bolt mass located around the barrel
Later design of the submachine gun, with most of the bolt mass located around the barrel

 

During WW1 and shortly afterwards, blowback action was also successfully applied to static guns, such as the 20mm Becker. Compared to other artillery pieces, these weapons were relatively low powered, and used deep chambers and rebated rim cartridges to contain pressure a while longer during the initial stages of the ejection cycle. Today, similar blowback actions are successfully employed in a number of 30mm and 40mm automatic grenade launchers such as the Russian AGS-17 or American Mk.19, which utilise relatively low-powered charges that generate only moderate pressures.

Despite some obvious advantages such as simplicity and low cost, blowback action does have noticeable drawbacks. Firstly, a blowback operated action is somewhat sensitive to the quality of the cartridge case material and the smoothness of the barrel chamber, to ensure reliable operation and avoid case ruptures or torn rims. Secondly, one has to balance the mass of the breechblock, the strength of the spring and the length of the breechblock cycle. If the spring is too weak or the bolt is too light, the resulting force will be insufficient to keep the cartridge inside the chamber while the pressure is high. This could cause the potentially disastrous situation of having a ruptured or separated case. If the spring is too strong, the gun will be difficult to cycle manually. If the bolt is too heavy, its movement will cause excessive vibration (swaying) while firing, nullifying any accuracy. Also, as the power of the cartridge (in this case, it’s internal pressure multiplied by the internal area of the cartridge base) grows, so grows the necessary weight of the bolt  to contain the cartridge in the chamber.

Normally, submachine guns that fire 9x19mm Luger ammunition have a breechblock weighing around 0.5 kg. With intermediate cartridges such as the 7.62×39 M43, the necessary breechblock weight rises to about 2 kg, and with a typical military rifle cartridge such as the 7.62×51 NATO the breechblock would theoretically weigh some 4 or 5 kilograms, making such systems absolutely impractical, not to mention their inherent unreliability due to the very high pressures involved.

With grenade launchers, which generate noticeably lower pressures on firing and are usually tripod- or vehicle-mounted, this is not a big deal. The increased dispersion, caused by a heavy breechblock oscillating back and forth inside the gun is also not an issue for weapons which are usually considered to be ‘area target’ rather than ‘point target’ guns.

As previously stated, the usefulness of blowback action is limited by the rearward pressure generated by firing the cartridge, dictating the weight of the breechblock and the strength of the spring. For handguns, which are normally limited in weight to 1 kg or less, the 9mm Luger round signifies the cut-off line for the application of simple blowback. While there were some practically working blowback pistols that fired 9x19mm ammunition, such as the now obsolete German HK VP70 or currently in production in the USA – the Hi-Point C-9; most blowback pistols fire relatively low-power ammunition, starting with 9×18 PM, 9×17 Browning and below.

Submachine guns, which can be shoulder fired weapons, can accommodate heavier bolts, and thus are produced with simple blowback actions in most ‘major’ pistol calibers, including 9×19, .40SW and .45ACP. No blowback operated rifle or machine gun, firing intermediate or full-powered military ammunition was ever produced “en masse”.