Definition: To "dry fire" a gun means to drop the hammer when the gun is not loaded with a live round of ammunition. This is usually not a good practice, because it can cause a lot of stress on parts of the gun, which causes undue wear and often leads to peening of parts and/or breakage. When firing live ammunition, the blow of the firing pin is cushioned somewhat by the primer or rim of the round being fired.
This is especially true of rimfire guns (22 long rifle, 22 magnum, 17 HMR, etc), because when dry-fired the firing pin on such guns will often damage the rear surface of the barrel, against which the cartridge rim lies when the gun is loaded.
Dry firing can be a useful practice for becoming familiar with a gun, and many firearms training regimens call for it, but it must be done properly in order to ensure that the gun is not damaged in the process. This can usually be accomplished by use of "snap caps," which are dummy cartridges designed to cushion the blow of the hammer and/or firing pin.
Examples:
It's not recommended practice to dry fire most firearms.
