A term used when you squeeze the trigger of a loaded gun
and the cap doesn't fire.
Priming caps (the gunpowder) over time are known to lose their "spontaneous" nature, particularly if stored improperly, causing what is known as a hang-fire: The primer smolders into a delayed ignition. So when you pull the trigger, instead of immediately firing, it could take place ten seconds later, or the thing could just snuff out. Roninspoon has helped me clarify this: Although primer caps often have black powder as a booster charge to ignite the smokless powder of the cartridge, the time senistive portion is the initiating percussion activated explosive, frequently Diazodinitrophenol.
If this happens, especially in older, antique weapons. Do NOT point the barrel anywhere else, as it could go off at any second. If you're at a shooting range and this occurs, keep it aimed at the target. I have never seen this happen, so I don't know what to do next. If you know about gun safety, you should know how to handle a gun, as if it is loaded and ready to fire.
There's an interesting story, over at The Darwin Awards, about this crook who stole a WWII gun from a home, and used it to rob a convenince store. As he left, he realized he didn't want to leave a witness. He aimed the gun at the clerk and pulled the trigger. Click. He had a hang-fire. Since the gun wasn't his, he probably had no clue about any of this, most people know glocks and revolvers.
Anyway, this clueless ID10T says "What the..." and looks down the barrel of the gun to see if the thing is loaded. I'm sure you can guess at where this is headed. Less than 5 seconds after he squeezed the trigger, the priming cap finally caught, and blasted the .45 caliber bullet straight through his skull, at over 900 feet/second.
I can't verify the authenticity of this story, but it came in through the grapevine, as well as at The Darwin Awards.
The story at: http://www.darwinawards.com/darwin/darwin1994-08.html