How Does It Work: Roller Delayed Blowback

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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.