Delayed blowback

Delayed blowback is a sub-class of blowback operation. In a "simple" blowback system, only the inertia of the bolt and the strength of its spring prevent opening of the action on firing; A delayed blowback system is not fully locked at the instant of firing, but its mechanism creates some form of additional mechanical resistance to the action opening, sometimes through temporarily locking part of the action just after firing. This allows gas pressure to drop and decreases operating pressures.

Comparison to other operating methods

Delayed blowback is by its nature slower, less reliable and more complex than simple blowback. It is more suited to firing high-powered rounds, as simple blowback requires the bolt to be impractically heavy with powerful cartridges: delayed blowback's mechanical resistance increases the effective weight of the bolt without actually making it heavier.

It has less precise manufacturing requirements than API blowback, though it also lacks the system's recoil-reducing effects and is generally not suited to autocannon-scale weapons. It also allows for a more accurate closed-bolt operation, while API blowback weapons must be open-bolt.

Compared to recoil operation, delayed blowback is somewhat more prone to malfunctions due to the lack of a positively locked breech on firing, has stronger felt recoil, and is more complex. However, it will typically allow for a faster rate of fire.

Compared to gas operation, delayed blowback actions are typically heavier, mechanically weaker, and require more complex manufacturing processes. They do have the advantage of allowing a free-floating barrel, while a gas-operated weapon will always have to have at least one additional component touching the barrel (the gas block). However, they lack the ability to adjust gas pressure easily for various temperatures or use of suppressors, high-powered ammunition or rifle grenades, while an adjustable gas port is a common feature on gas-operated weapons.

In general delayed blowback actions tend to extract cartridges very forcefully, and are thus not well-liked by civilian shooters who recover and reload spent brass.

Types

Lever delayed

Animation showing the cycle of a lever-delayed blowback action.

Lever-delayed blowback operation uses a lever to delay the opening of the bolt. As the action opens, a lever mounted to the side of the bolt head interfaces with the bolt carrier, the top of the lever forcing the bolt carrier backwards before it can tilt enough to allow the action to open fully.

Roller delayed

Roller-delayed blowback operation uses rollers on the sides of the bolt head: as the bolt head is driven back these rollers swing out into slots in the receiver and temporarily prevent rearward motion of the bolt head, while the bolt carrier is allowed to proceed rearwards.

Gas delayed blowback

Gas-delayed blowback operation is where propellant gas tapped from the barrel is used to delay the opening of the bolt, typically through use of a piston.

Hesitation locked

Hesitation locked blowback uses a breech block that holds the bolt closed until the gas drops to a safe level to extract.

Primer actuated

Primer actuated blowback is where the primer moves rearwards to open the bolt.

Screw delayed

Screw delayed blowback is where the bolt has angled lugs that have to rotate in the manner of a screw before the action can open.

Flywheel delayed

Flywheel delayed blowback operation, where during firing the bolt opening is delayed by the rotational inertia of a flywheel, which are driven by a rack and pinion arrangement on the bolt carrier.

Radial delayed

Radial delayed blowback operation uses the rotation of the bolt head to accelerate the bolt carrier.

Off-axis

The bolt's travel path is in a different direction to the barrel axis.

Chamber ring

Chamber ring delayed blowback uses the cartridge pressure to hold the round in the chamber.

Toggle delayed

Toggle delayed blowback uses the rearward moving bolt to overcome a hinged joint.

Locking lug delayed

Locking lug delayed blowback uses a lug to hold the bolt.

Detent delayed

Detent delayed blowback uses a spring loaded detent installed inside the bolt that locks itself into a notch on the end section of the guide rod closest to the barrel chamber rather than a section of the receiver/trunnion.