How stuff works
There's a cool site, made by Discovery that shows in detail "how stuff works", including many kinds of firearms. Take a look at it here. It's a good compendium of knowledge, taking advantage from web technology to show them in motion. What I didn't like on normal sites explaining firearms - though it's important whan actually using firearms - is that they talk so much about safety and all the boring stuff. Howstuffworks does really focus on how it works. That's a good thing if you just want to know how it works. Firearms are only a fraction of the knowledge covered there. It always useful to keep this site in mind or bookmarked - especially if you want to build something.