The best sources for the general level are those "coffee table" books you can often find "remaindered" at the book stores. Most of the books I have, for example, are at a much higher (or lower) level from a general overview of the type in a term paper.
Also, Wikipedia provides a great source for that kind of stuff. (Be careful in quoting - profs also know about Wikipedia and almost always will look for plagiarism.)
One thing, watch terminology; you just might have a prof who does know something about guns. A cartridge is loaded into a gun; a bullet comes out of the barrel. A rifle has grooves, called "rifling", in the barrel to spin a bullet and give it gyroscopic stability. Shotguns and old muskets are smooth bored, without those grooves.
Jim