Does anyone know where I can find information on various machine guns/machine pistols? I'm not planning on getting one, but I'd like to know more about them, things like which companies make what, specs, how they work, etc. It seems like this is a little tougher to find information on, probably since they are hard to get and not much more useful then the semi-auto versions, but I'm just curious.

Ever see those big tables at the gun shows with nothing but books?  A goldmine of info . . .

The standard reference work for all things military gun, if you had to pick one, is Small Arms of the World.  That tells you how most all significant FA small arms work.

In addition to CR's advice, you should get yourself a subscription to Small Arms Review.