As promised/threatened, I did a little more research into gun mechanisms. It looks like there's a bit more variation in the way that different types of guns reload themselves, so we'll stick to a basic semiautomatic pistol action.Bullets for a typical semiautomatic pistol are loaded into a clip or magazine, which is basically just a box, open at the top, with a spring on the bottom. (Apparently "clip" is not the technical term, and is supposed to refer to something else, but people use them fairly interchangeably, so whatever). As a bullet is taken off the top, the spring pushes another bullet up into its place. Basically, it's a really dangerous Pez dispenser.
When the cartridge fires, most of the force of the explosion is channeled into pushing the bullet forwards, but a little bit of it goes backwards instead. This pushes a slide that rests along the top of the gun backwards, which does a couple of things. First, it carries the empty cartridge backwards until it hits the back end, which usually has a little flange or something that pops the cartridge out sideways, clearing the chamber. Second, the motion puts energy back into the spring that drives the firing pin/hammer, readying it for another shot.
Finally, as the slide moves the spent cartridge backwards, the way is cleared for the bullet at the top of the magazine to be pushed up into the chamber and into firing position, a second spring pulls the slide back into place, and the whole cycle is ready to start again.
