You are not likely to discharge most modern guns by dropping them, but there are consequences. You can scratch the paint, dent the frame, gouge out some metal, or push a piece out of alignment. If you drop your gun, inspect it for damage. Rack the slide on a semi-automatic, check the alignment of the hammer, and check for dirt in the action. If anything is amiss, take it to a gunsmith. Firing it damaged can do further damage to the gun or to you.