I'm a civilian, and I know almost NOTHING about military guns. (like which type different guys in each branch use, what they're like, what they shoot, what are their strong and weak points, etc.) Does anyone know where a good source to find this information is?

http://www.recguns.com/

Hope that helps, a lot of the information is incomplete, but it should give you a basic idea.

Here is another really good site.
http://www.world.guns.ru/main-e.htm

an ar-15 is the best choice in my opinion. An 60e is pretty good but has little proficiency.