Gas Retarded

A type of delayed blowback operation in which a portion of the powder gasses is bled off from the barrel to retard the rearward travel of the slide--used in the Heckler & Koch P7 series of 9mm pistols. The P7 is a semi-automatic blowback-operated firearm. It features a unique gas-delayed blowback locking system modeled on the Swiss Pistole 47 W+F (Waffenfabrik Bern) prototype pistol (and ultimately on the Barnitzke system first used in the Volkssturmgewehr), which used gas pressures from the ignited cartridge and fed them through a small port in the barrel (in front of the chamber) to retard the rearward motion of the slide. This is accomplished by means of a piston contained inside of a cylinder located under the barrel that opposes the rearward motion of the slide until the gas pressure has declined—after the bullet has left the barrel—hence allowing the slide to end its rearward motion, opening the breech and ejecting the empty cartridge case.

Gun Gas Retard Blow Back
A schematic of the P7's gas-delayed blowback system.