Gas-delayed blowback operation is where the gas propellant is used to delay the opening of the bolt. When firing, the bolt is never locked and moves backwards like a blowback firearm, only for the gas to be vented out the barrel into a piston that delays the bolt.
Development[]
The operation was used by some World War II German designs, notably the Volkssturmgewehr 1-5 rifle and the Grossfuss Sturmgewehr, and later on after ww2 by the Heckler & Koch P7, Steyr GB and M-77B pistols.