How Does It Work: Roller Delayed Blowback

Roller delayed blowback originated in the Mauser company R&D department when engineers were attempting to design a roller-locked version of the G43 rifle. They found that when the bolt bounced in automatic fire, the system would sometimes work without being fully locked. With some refinement, this became the roller delayed system, and would have been used in the StG-45 rifle if the war had gone longer. After the war, the system was applied to prototype French arms, then to the Spanish CETME program which eventually became the German G3 rifle. The system would see use primarily as the basis of a whole family of arms from Heckler & Koch, although it has been used in a few other places.

Mechanically, the system uses an angled “locking” wedge to put a mechanical disadvantage on a pair of rollers that must retract into the bolt head before it can move rearward. The combination of the wedge angle and the mass of the bolt carrier assembly are carefully calculated to delay the action from opening until pressure is reduced to a safe level. These systems do typically open faster than locked actions, though, and generally require the use of chamber fluting to ensure reliable extraction.


  1. What happened to the HK 25 50 cal machine gun? Is it even possible to create such a thing with a roller-delayed blowback mechanism?
    IMHO yes. What do you think?

    • And what can happen to something that does not exist?
      I have not seen a single intelligible mention of this machine gun.
      Of course, if you do not count any Internet sites with coloring pictures for kindergarten. 😉

      And to build one, I see no obstacles.
      Just triple the MG21.

  2. Both lever and roller delay blowback meshanisms can not be explained through drawings and videos. Both are based on receiver supported levers forcing greater massed bolt carrier back through mechanical disadvantage (applying more force with lesser leverage) and by means, wasting most of recoil force to accerelate it and since the recoil force obtained from a cartridge being fixed, giving lesser amount of remained momentum to the bolt as slowing its opening speed. Dlfference of roller delay from lever being that, the rotating axis of the former is movable to a zero point. lMHO…

  3. Come on…
    What could be incomprehensible there?
    In the scheme with a roller delayed return, a “dummy mass” works.
    This means that the theoretical mass of the mobile group increases as many times as the acceleration of the carrier relative to the bolt increases. The roller, which is sandwiched between the surfaces of the carrier, bolt and body, tends to “shoot” like a cherry bone from the fingers. While pushing the carrier.

    And in the scheme with positive roller locking, the rollers are supported by one half in the cutout of the bolt, and the other half in the cutout of the housing. And they are held in this position until the middle part of the carrier bursting them does not allow them to get closer and exit the cutouts of the case.

  4. Yes. I almost forgot.
    With positive locking (MG42) during the time when the unlocking occurs, the rollers, as in the delayed circuit, work as a carrier accelerator.

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