Blowback (firearms)
Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge.
Several blowback systems exist within this broad principle of operation, each distinguished by the methods used to control bolt movement. In most actions that use blowback operation, the breech is not locked mechanically at the time of firing: the inertia of the bolt and recoil, relative to the weight of the bullet, delay opening of the breech until the bullet has left the barrel. A few locked breech designs use a form of blowback to perform the unlocking function.
The blowback principle may be considered a simplified form of gas operation, since the cartridge case behaves like a piston driven by the powder gases. Other operating principles for self-loading firearms include delayed blowback, blow forward, gas operation, and recoil operation.
Principle of operation
In firearms, a blowback system is generally defined as an operating system in which energy to operate the firearm's various mechanisms, and automate the loading of another cartridge, is derived from the inertia of the spent cartridge case being pushed out the rear of the chamber by rapidly expanding gases produced by a burning propellant, typically gunpowder. When a projectile is still within the gun barrel, the high-pressure propellant gas behind it is contained within what could be seen as a closed system, but at the moment it exits the muzzle, this functional seal is broken, allowing the propellant gas to be suddenly released in an explosive muzzle blast. The expanding gas also creates a jet propulsion effect rearward in the barrel against the spent cartridge case. This "blowback" is the predominant component of the recoil. Some guns use energy from blowback to perform the automatic bolt cycling /reloading process, while others will use a portion of the blowback to operate only certain parts of the cycle or simply use the blowback energy to enhance the operational energy from another system of automatic operation.What is common to all blowback systems is that the cartridge case must move under the direct action of the powder pressure; therefore, any gun in which the bolt is rigidly locked will undergo a degree of blowback action. The energy from the expansion of gases upon firing appears in the form of kinetic energy transmitted to the bolt mechanism, which is controlled and used to operate the firearm's operation cycle. The extent to which blowback is employed largely depends on the manner used to control the movement of the bolt and the proportion of energy drawn from other systems of operation. How the movement of the bolt is controlled is where blowback systems differ. Blowback operation is most often divided into three categories, all using residual pressure to complete the cycle of operation: "simple blowback", "delayed/retarded blowback", and "advanced primer ignition".
Relating blowback to other types of automatic firearm operation, George M. Chinn wrote that: "In the larger sense, blowback might well be considered a special form of gas operation. This is reasonable because the cartridge case may be conceived of as a sort of piston driven by the powder gases. Actually, blowback involves so many special problems that it is best considered to be in a class by itself. The question whether or not it should be included within the more general class of gas operation or recoil operation is purely academic. The important point is that it partakes some of the properties of both classes and, depending on the particular problem at hand, may be considered to be either one."
History
In 1663 a mention is made in the journal of the Royal Society for that year of an engineer who came to Prince Rupert with an automatic weapon, though how it worked is unknown. In 1854 a hydropneumatically delayed-blowback cannon was patented by Henry Bessemer. In 1856 a crank-operated cannon with a blowback-operated cocking mechanism was patented in the US by Charles E. Barnes. In 1876 a single-shot breech-loading rifle with an automatic breech-opening and cocking mechanism using a form of blowback was patented in Britain and America by the American Bernard Fasoldt. In 1883 Hiram Maxim patented a blowback-operated rifle. In 1884 he would also patent a toggle-lock delayed-blowback-operated rifle. Also in 1884, a few months after Maxim, a British patent for blowback-operated pistols and rifles was filed by Richard Paulson. In 1887 a patent was filed by an American inventor called Carl J. Bjerkness for a blowback-operated rifle. In 1888 a delayed-blowback machine gun known as the Skoda was invented by Grand Duke Karl Salvator and Colonel von Dormus of Austria.Simple blowback
The blowback system represents the most basic auto loading operation type. In a blowback mechanism, the bolt rests against the rear of the barrel, but is not locked in place. At the point of ignition, expanding gases push the bullet forward through the barrel while at the same time pushing the case rearward against the bolt. The expanding gases push the bolt assembly to the rear, but the motion is slowed by the mass of the bolt, internal friction, and the force required to compress the action spring. The design must ensure that the delay is long enough that the bullet exits the barrel before the cartridge case clears the chamber. The empty case is ejected as the bolt travels to the rear. The stored energy of the compressed action spring then drives the bolt forward. A new cartridge is stripped from the magazine and chambered as the bolt returns to its in-battery position.The blowback system is practical for firearms using relatively low-power cartridges with lighter weight bullets. Higher power cartridges require heavier bolts to keep the breech from opening prematurely; at some point, the bolt becomes too heavy to be practical. For an extreme example, a 20 mm cannon using simple blowback and lubricated cartridges would need a bolt to keep the cartridge safely in the barrel during the first few milliseconds. Yet the bolt must cycle far enough back to eject the spent casing and load a new round, which would limit the return spring to an average force of. The resulting system, if it could be built, would not have enough energy to cycle reliably or even keep the bolt closed when the gun is tilted up.
Due to the required bolt weight, blowback designs in pistols are generally limited to calibers smaller than 9×19mm Parabellum There are exceptions such as the simple blowback pistols from Hi-Point Firearms which include models chambered in.40 S&W and.45 ACP. Simple blowback operation can also be found in small-bore semi-automatic rifles, carbines and submachine guns. Most simple blowback rifles are chambered for the.22 Long Rifle cartridge. Popular examples include the Marlin Model 60 and the Ruger 10/22. Most blowback carbines and submachine guns are chambered for pistol cartridges such as the 9×19mm Parabellum,.40 S&W and.45 ACP. Examples include the MP 40, Sten and UZI. The bolt can be made bigger and more massive in these weapons than in handguns, as they are intrinsically heavier and designed, ideally at least, to be fired with both hands, often with the aid of a shoulder stock; and these factors help to ameliorate the disruption to the shooter's aim caused by the heavy bolt's movement. Consequently, simple blowback is adequate for somewhat more powerful rounds in submachine guns than in standard pistols. The barrel usually requires a short length in simple blowback firearms as this is to prevent rupturing cartridges. One of the very few known simple blowback firearms capable of firing fully powered rifle cartridges was the Brixia 930 light machine gun, that required a large bolt to handle the pressure of the round as well as a spring buffer shock absorbing butt plate on the stock to handle recoil. There were also a few rifles that chambered cartridges specifically designed for blowback operation. Examples include the Winchester Model 1905, 1907 and 1910. The only known assault rifle to use simple blowback was the Burton Model 1917.
Although simple blowback is limited to guns using low-power rounds, it is so efficient that in small-calibre semi-automatic pistols it has become almost ubiquitous. Heavier calibre semiautomatic handguns typically employ a short recoil system, of which by far the most common type are Browning-derived designs which rely on a locking barrel and slide assembly instead of blowback. But blowback guns can be used to fire powerful cartridges if they are of the other two types: API or delayed blowback.
Advanced primer ignition (API) blowback
In the API blowback design, the primer is ignited when the bolt is still moving forward and before the cartridge is fully chambered. This requires a very careful design to ensure the proper balance and equalization of forces between the projectile weight, propellant charge, barrel length, bolt weight, and return spring strength. In a simple blowback design, the propellant gases have to overcome static inertia to accelerate the bolt rearwards to open the breech. In an API blowback, they first have to do the work of overcoming forward momentum to arrest the forward motion of the bolt. Because the forward and rearward speeds of the bolt tend to be approximately the same, the API blowback allows the weight of the bolt to be halved. Because the momentum of the two opposed bolt motions cancels out over time, the API blowback design results in reduced recoil.Advanced primer ignition was originally developed by Reinhold Becker for use on the Becker Type M2 20 mm cannon. It became a feature of a wide range of designs that can be traced back to Becker's, including the Oerlikon cannon widely used as anti-aircraft weapons during World War II.
To increase performance of API blowback firearms, larger calibre APIB guns such as the Becker and Oerlikon use extended chambers, longer than is necessary to contain the round, and ammunition for APIB firearms come with straight-sided cartridges with rebated rims. The last part of forward motion and the first part of the rearward motion of the case and bolt happen within the confines of this extended chamber. As long as the gas pressure in the barrel is high, the walls of the case remain supported and the breach sealed, although the case is sliding rearwards. This sliding motion of the case, while it is expanded by a high internal gas pressure, risks tearing it apart, and a common solution is to grease the ammunition to reduce the friction. The case needs to have a rebated rim because the front end of the bolt will enter the chamber, and the extractor claw hooked over the rim therefore has to fit also within the diameter of the chamber. The case generally has very little neck, because this remains unsupported during the firing cycle and is generally deformed; a strongly necked case would be likely to split.
The API blowback design permits the use of more powerful ammunition in a lighter gun than would be achieved by using simple blowback, and the reduction of felt recoil results in further weight savings. The original Becker cannon, firing 20×70mmRB ammunition, was developed to be carried by World War I aircraft, and weighed only 30 kg. Oerlikon even produced an anti-tank rifle firing 20×110mmRB ammunition using the API blowback operation, the SSG36. On the other hand, because the design imposes a very close relationship between bolt mass, chamber length, spring strength, ammunition power and rate of fire, in APIB guns high rate of fire and high muzzle velocity tend to be mutually exclusive. API blowback guns also have to fire from an open bolt, which is not conducive to accuracy and also prevents synchronized fire through an aircraft propeller arc.
According to a United States Army Materiel Command engineering course from 1970, "The advanced primer ignition gun is superior to the simple blowback because of its higher firing rate and lower recoil momentum. However, favorable performance depends on timing that must be precise. A slight delay in primer function, and the gun reverts to a simple blowback without the benefit of a massive bolt and stiffer driving spring to soften the recoil impact. The exacting requirements in design and construction of gun and ammunition reduce this type almost to the point of academic interest only."
API mechanisms are very sensitive to the ammunition used. For example, when the Germans switched their MG FF to their new, lighter mine shell, they had to rebalance the spring strength and bolt weight of the gun, resulting in a new MG FF/M model with ammunition not being interchangeable between the two models. The 30 mm MK 108 cannon was perhaps the apogee of API blowback technology during World War II.
The principle is also used in some automatic grenade launchers, for example in the US Mk 19 grenade launcher or Russian AGS-30.