This can only work with certain kinds of cap guns. The ones you find at disneyland. They look like old single shot black powder guns. They have usually only one spot for a cap(somtimes two depending on what kind of gun it is). This is very simple to do.
All you do is where you would normally put your cap, you put about 1 – 2 firecrackers worth of flash powder. Make sure it is spread out evenly. Then you place two to three caps on top of the powder. This is made so when you pull the trigger and the hammer drops it will set of the caps and the caps with thier violent flash will ignite the flash powder instintainiously causing a loud BOOM!!
I have heard mine go let off a sound equivalent to a 9mm pistol. The average is about as loud as a 22 pisol, but varies on a few things (how much powder, how many caps, the rate of the hammer, and how fast the caps go off). You can not set this of with just the hammer by it self because it takes alot more pressure to ignite the caps since the powder is cusioning it. This is done very simply.
Just add 5 to 6 rubber bands around the hammer to give it more force as it flies down. If you shoot the gun off right after you load it you can leave it like that, if not you must put a piece of tape over the powder so it does not get blow off. This will provide fun for you and a fake threat to your loser neighbors.