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How a HK P7 works
The P7 is a German 9×19mm semi-automatic pistol designed by Helmut Weldle and produced by Heckler & Koch GmbH (H&K) of Oberndorf am Neckar. It was revealed to the public for the first time in 1976 as the PSP (Polizei-Selbstlade-Pistole—"police self-loading pistol")
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 1-5), 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 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.
The chamber has 18 flutes that aid in the extraction process by allowing combustion gases to flow between the fired case and the chamber walls, preventing the case from "sticking" to the chamber walls. The drawback of this system is that the breech "opens" slightly prematurely to allow the slide to initiate its rearward motion. The high temperature gases cycling through a tube located below the chamber area and above the trigger made the early versions of this pistol uncomfortable, according to some, to shoot after the content of two magazines were fired due to heating. The advantages of this system are a simpler manufacturing process due to the absence of a locking system and a high mechanical accuracy due to the barrel being fixed in the frame; the barrel does not execute any sort of lateral or vertical movement during the operating cycle as with the Browning cam-action systems common to many other locked breech pistols.