How the MP5’s Roller-Delayed Blowback Action Works

The MP5, one of the most infamous SMG platforms ever devised, hinges largely on the success of its action. Known as a roller-delayed blowback operating system, the MP5’s action is an object of interest among shooters, collectors, firearms enthusiasts, historians, and others.

Many semi-automatic pistols and submachine guns use a direct blowback operation to cycle the action. Direct blowback, which is one of the simplest types of autoloading operations, uses the pressure created from gasses produced during the deflagration of the propellant charge to cycle the action.

In direct blowback operation, when the bullet is propelled through the barrel under the influence of expanding gasses, the slide and breech move back, extracting and ejecting the spent casing and feeding a new cartridge into the chamber.

Direct blowback actions are serviceable, but they are dirty and produce a large amount of recoil. The MP5’s roller-delayed blowback action is superior.

In a roller-delayed blowback system, the breech is not locked shut with lugs. Instead, the bolt face is held shut against the breech through the bolt mass and spring pressure, bolstered by a set of rollers on the MP5 bolt carrier group that fit into recessions along the barrel trunnion. So it is not fully “locked,” but it is held in place by a greater force than just bolt mass and spring pressure.

When a round is fired, pressure builds inside the chamber and barrel, pushing the projectile along the barrel. Pressure behind the bullet also presses against the bolt face. In a direct blowback system, this would force the bolt to the rear, but in a roller-delayed blowback system, the semi-locking action of the rollers in the trunnions keeps the bolt shut for longer, delaying the rearward impetus of the bolt.

At a certain point, gas pressure gets high enough to overcome the resistance of the rollers in the trunnions, forcing the bolt backward and the rollers to retreat into the sides of the bolt, allowing the bolt carrier group to extract and eject the spent cartridge before the recoil springs complete the cycling of the action, feeding a new round into the chamber.

This sort of action has a number of advantages over direct blowback, delayed blowback, and certain other gas-operated autoloading systems.

Advantages of a Roller-Delayed Blowback Action
The use of rollers to detain and delay the bolt carrier group means that (potentially) weaker spring and lighter bolt carrier groups can be used in the construction of firearms with these types of actions.

Since a lighter bolt may be used to create a roller-delayed blowback action, felt recoil falls substantially as a result. This means that with hard-kicking cartridges, shooter fatigue can be diminished. The lower recoil also generates less muzzle flip, which can be a serious obstacle for handguns and other weapons cycled through direct blowback.

The other great thing about a roller-delayed blowback action is that it allows chamber pressure to drop to lower levels before the bolt begins to cycle. This cuts back on port pop and can vastly improve suppression.

These systems, despite the fact that they can’t be effectively tuned, are simpler than many recoil-operated or gas-impingement actions. There is also no gas system, tube, or block to clean with a roller-delayed blowback system – although fouling must still be diligently removed to ensure smooth cycling.

Need a New MP5 Bolt Carrier Group? Visit Zenith Firearms
Looking for an MP5 bolt carrier group, firing pin, or other internal components for your ZF-5 or some other MP5 clone? Visit Zenith Firearms online at

In addition to MP5 bolt carrier groups, they carry a wide range of MP5 parts and accessories, including slings, magazines, and muzzle devices. Visit their website for more information or contact them at 434-202-7790.

For more information about Mp5 Thread Protector and American Made Mp5 Please visit: Zenith Firearms.

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