[Alpo]

It could be humidity, or it could just be the roof leaks, and the drip happens to fall on the old T-shirt the gun is wrapped in.

If you want it to go click, and then work, the easiest thing would be to make it a revolver. Assuming you know nothing about guns. See the round thing above the trigger in the top picture? That the cylinder. It holds the cartridges (from five to upwards of 20, depending on the gun), and as the hammer goes back the cylinder rotates, bringing a loaded chamber in line with the barrel. The hammer falls, the gun goes bang, when the hammer is cocked again the cylinder rotates once more, moving that chamber out of the way and bringing another (hopefully loaded) chamber in line with the barrel. You could either have some of the ammo be contaminated, so the first round or two just went click - dead shells - but then try #3 went bang. Or, you could have the ammo stored with the gun, but not in it, and not have enough to fully load it. If your hero is not thoroughly up on guns he could maybe not realize that S&W rotates counterclockwise while Colts rotate clockwise. If, for example, there were only 4 shells, he might load the gun so the first four, going counterclockwise, would fire, but the gun, being a Colt, turns clockwise, and the first trigger pull would result in a click, on the empty chamber.