Blowback (firearms) - Simple Blowback

Simple Blowback

The simple (sometimes referred to as the "straight" or "pure") blowback system represents the most basic form of blowback operation and demonstrates the basic principles involved in the blowback cycle. The simple blowback mechanism typically consists of the bolt which rests against the base of the cartridge case and a recoil spring that is compressed by the kinetic energy of the bolt when it is thrown back in recoil. The stored energy of the compressed spring then drives the bolt back forward into firing position.

The cycle begins when the cartridge is fired. With an open-bolt cycle, the bolt is held by the trigger sear to the rear and the recoil spring is fully compressed. Pulling the trigger releases the sear, the action spring then propels the bolt forward, which strips a round from the feed system along the way. The bolt carries a new cartridge into the chamber with considerable velocity and at the end of its travel the firing pin fires the primer, igniting the propellent. The pressure of expanding gases from the propellant sends the projectile down the barrel and at the same time applies an opposite, rearward, force to the shell case against the breech face of the bolt, accelerating the bolt and casing rearward with a force equal to F = ma, where "m" is the mass of the bolt and casing, and "a" is the rate of acceleration of the bolt (the resistance of the recoil spring can be considered to be negligible until the bore pressure drops). The force is also equal to F = PA, where "P" is the instantaneous gas pressure inside the bore, and "A" is the cross-sectional area of the chamber (the pressure force and inertia force are equal and opposite, i.e. same "F" but in opposite directions). The breech is kept sealed by the internal pressure of the cartridge case against the chamber until the bullet has left the barrel; the inertia of the bolt mass ensures this (mass of the bolt + recoil spring, in some cases the hammer force too). At this point the bore pressure is zero and the force driving the bolt back is also zero, but the case and bolt continue to the rear on their own momentum. At the moment the bullet leaves the barrel, the momentum of the bullet and the rearward travelling bolt are equal and opposite, assuming a same diameter bore and chamber, such as the sten gun (which is not true with delayed blowback systems in which some of the momentum is initially transferred directly to the bulk of the gun, or with necked cartridges where the casing is a larger diameter than the projectile). The momentum of the bolt is gradually transferred to the body of the gun and the shooter's body as the recoil spring is compressed. As the bolt travels back, the spent cartridge case is extracted and ejected, and the firing mechanism is cocked by the rearward travelling bolt. The bolt eventually reaches a velocity of zero and the kinetic energy from the recoil impulse is now stored in the fully compressed spring (some energy loss does occur due to friction and the extraction and ejection sequences). The cycle repeats until the last round is expended or the trigger is released engaging the sear to hold the bolt in the rear (open-bolt) position.

To remain practical, this system is only suitable for firearms using relatively low pressure cartridges. Pure blowback operation is typically found on semi-automatic, small-caliber pistols, small-bore semi-automatic rifles and submachine guns. Some low-velocity cannon and grenade launchers such as the Mk 19 grenade launcher also use blowback operation.

The barrel of a blowback pistol is generally fixed to the frame and the slide is held against the barrel only by the recoil spring tension. The slide starts to move rearward immediately upon ignition of the primer. As the cartridge moves rearward with the slide, it is extracted from the chamber and typically ejected clear of the firearm. The mass of the slide must be sufficient to hold the breech closed until the bullet exits the barrel and residual pressure is vented from the bore. A cartridge with too high a pressure or a slide with too little mass may cause the cartridge case to extract early, causing a separation or rupture. This generally limits blowback pistol designs to calibers less powerful than 9x19mm Parabellum (e.g., .25 ACP, .32 ACP, .380 ACP, 9x18mm Makarov, etc.). Any larger and the slide mass starts to become excessive, and therefore few blowback handguns in such calibers exist (see Recoil operation (Short recoil operation) for the method most commonly used by these pistols); the most notable exceptions are simple, inexpensive guns such as those made by Hi-Point Firearms which includes models chambered in .45 ACP, .40 S&W, .380 ACP and 9x19mm Parabellum.

Most simple blowback rifles are chambered for the .22 Long Rifle cartridge. Popular examples include the Marlin Model 60 and the Ruger 10/22. Some blowback rifles or carbines are chambered for pistol cartridges, such as the 9mm Parabellum, .40 S&W and .45 ACP. Examples include the Ruger Police Carbine, Beretta Cx4 Storm, Marlin Camp Carbine and Hi-Point Carbine. There were also a few rifles that chambered cartridges specifically designed for blowback operation. Examples include the Winchester Model 1905, 1907 and 1910. A very unusual blowback rifle was created by fitting the M1903 Springfield rifle with a mechanism called the Pedersen device in an attempt to turn a bolt-action rifle into a semi-automatic repeater with essentially a pistol cartridge.