sentences:  Others use either the firearm's recoil or a small portion of the propellant gas drawn from the barrel, to operate the firearm's mechanism and ready it for the next shot.Such firearms are sometimes called "self-loading," but are more commonly known as semi-automatic, if they fire one shot for every pull of the trigger, or automatic or "full-auto" if they continue to fire until the trigger is released or the magazine is empty.A revolver is a unique type of firearm in which a rotating cylinder holds a number of cartridges; the cylinder "revolves" to align each "chamber" or "charge hole" with the rear of the barrel, hold the cartridge and contain the pressure (up to 65,000 pounds PSI or 450 MPa) produced when the cartridge is fired.