the exact arrangement of the hammer changes from gun to gun, i believe. in an older gun, like a revolver, it's cocked by manually pulling it back (but since it moves in an arc, that also means that it goes up), then released when you pull the trigger (so it swings down and forwards).
other guns have a hammer that slides back and forth horizontally on the end of a spring.
basically, the hammer will move differently and be shaped differently, depending on the design of the gun, allowing for different types of bullets, different firing rates, etc.
the basic mechanism is the same, though, in just about any modern gun -- something hits the back of the bullet. as some people at camp discovered when they stole a bullet from the riflery range (we got to shoot cans with bb guns once a week, but they also had a real rifle for the adults), you don't even need to hit it with anything special. they hit the back of the bullet with a real hammer, and it worked just fine. well enough that the kid who happened to be standing in the wrong place (a few feet away) at the wrong time got his ear pierced.
personally, i've always figured that it was the bullet that was rigged rather than the gun. a small electric charge should set off the bullet just as well as a physical blow.
so, a quick review of bullet design...
at the front, you have a metal cone. that's what is actually launched. behind the cone, you have a cylinder full of gunpowder. the far end of the cylinder is, of course, a circle. that's the back of the bullet. inside the circle is a smaller circle which is the end of a small blasting cap (a little metal can filled with something that will explode fairly easily when you hit it or run a spark through it).
when that little cap explodes, it sets off the gunpowder, which results in a larger explosion that forces the bullet (or at least the front of the bullet, depending on the bullet's design) out through the barrel of the gun. since the barrel is fairly small, the air pressure after the explosion remains fairly high as the bullet travels down the barrel. as long as the bullet is still in the barrel, it's still being pushed by the pressure of the explosion, and is thus accelerating (which is why a gun with a longer barrel will shoot a bullet farther than a gun with a short barrel).
so, to set off a gun remotely, all you need to do is set off that little blasting cap (it probably has a name, but i don't know it offhand) at the back of the bullet. you can either rig the gun with some kind of motor that will pull the hammer back and make it hit the bullet or, far easier, you could set the bullet off with a spark.
now, if you have a radio signal, any capacitor set with the distance between the plates equal to the wavelength of the signal will pick up that signal and convert it to an electric current. that's how most radios work. when you adjust the dial, you're changing the distance between the plates, thus changing the wavelentgh you're picking up. (more info on basic radio recievers can be found
here. )
so, if you had a remote that would send out a single radio pulse at a preset wavelength, and you had something on the back of the bullet (inside the blasting cap or directly attached to it) that had a radio reciever set to pick up that exact wavelength, and the remote was near enough and powerful enough, you'd have a spark in the right place powerful enough to set off the bullet. it just takes a bit of work to put it in place, and all the parts are readily available at, say, radio shack.
you could either do that to a single bullet (bury the reciever inside the bullet, inside the blasting cap, or on the back of the bullet) or you could replace the hammer of the gun with the transistor set-up.
press the button, send out the signal, and zap! bang! off goes the bullet!
hope that helps.
Paul
