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PRINCIPLE In a blowback mechanism, the mass of the bolt and force of the recoil spring act to keep the breech closed as the cartridge is fired. Expanding gases from the fired round overcome this inertia and "blow back" the breech. The breech is kept sealed by the cartridge case until the bullet has left the barrel and gas pressure drops to a safe level; the weight of the bolt and spring pressure ensure this. To remain practical, this system is only suitable for weapons using relatively low pressure rounds. Pure blowback operation is typically found on semi-automatic small-caliber Pistol s and automatic Submachine Gun s. Some low-velocity Cannon or grenade launchers such as the Mk 19 Grenade Launcher use blowback operation. DELAYED BLOWBACK SYSTEMS For more powerful rounds and for a lighter operating mechanism, some system of delayed or '''retarded''' blowback is often used requiring the bolt to overcome some initial resistance while moving. Because of high pressures, delayed blowback firearms such as the G3 rifle have fluted chambers to ease extraction. There are various delay mechanism: Roller delayed Heckler & Koch refined roller-delayed blowback operation from the distinctly different roller-locked Recoil Operation as seen in the MG42 . Unlike the MG42, in roller-delayed blowback the barrel is fixed and does not recoil. As the bolt head is driven rearward, rollers on the sides of the bolt are driven inward against a tapered bolt carrier extension. This forces the bolt carrier rearward at a much greater velocity and slows movement of the bolt. The primary advantage of roller-delayed blowback is the simplicity of the design compared to gas or recoil operation.Stevens, R. Blake. (2006). Full Circle, A Treatise on Roller Locking. Collector Grade Publications. ISBN 0889354006 Roller-delayed blowback was first used in the experimental MG 42 derivative MG42V and the 1945 Mauser Sturmgewehr 45 prototype. Ther first full-scale production rifle to utilize roller-delay was the Spanish CETME . Modern examples are the Heckler & Koch G3 Rifle , MP5 Submachine Gun , and P9 Pistol as well as the Sturmgewehr 57 . Lever delayed Lever-delayed blowback utilizes leverage to delay the opening of the breech. When the cartridge pushes against the bolt face, the lever moves the bolt carrier rearward at an accelerated rate relative to the light bolt. This leverage significantly increases resistance and slows the movement of the lightweight bolt. The mechanism was invented by and the AA-52 Machine Gun . Gas delayed Gas-delayed blowback should not be confused with Gas-operated . The Bolt is never locked, and so is pushed rearward by the expanding Propellant gases as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt. It is used by Volkssturmgewehr 1-5 rifle, the Heckler & Koch P7 and Steyr GB pistols. Chamber-ring delayed When a cartridge is fired, the case expands to seal the sides of the chamber. This seal prevents high-pressure gas from escaping into the action of the gun. Because a conventional chamber is slightly oversized, an unfired cartridge will enter freely. In a chamber-ring delayed firearm, the chamber is conventional in every respect except for a raised portion at the rear of smaller diameter than the front of the chamber. When the case expands in the front of the chamber and pushes rearward on the slide, it is slowed as this raised portion constricts the expanded portion of the case as the case is extracted. The Seecamp pistol operates on this principle. Hesitation locked John Pedersen 's patented system uses a separate breech block within the slide or bolt carrier. When in battery, the breech block rests slightly forward of the locking shoulder in the frame. When the cartridge is fired, the bolt and slide move together a short distance rearward powered by the energy of the cartridge as in a standard blowback system. When the breech block contacts the locking shoulder, it stops, locking the breech in place. The slide continues rearward with the momentum it acquired in the initial phase. This allows chamber pressure to drop to safe levels while the breech is locked and the cartridge slightly extracted. Once the bullet leaves the barrel and pressure drops, the continuing motion of the slide lifts the breech block from its locking recess through a cam arrangement, continuing the firing cycle. The Remington 51 was the only production pistol to utilize this type of operating system. Toggle delayed |
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