I find myself referring to WGC's and UNCompanies info links for gun diagrams occasionally. Here are some links to pages with gun schematics and technical manuals:
http://www.uncompany.com/pageinfodiagram.asp#guide
http://www.wargameclub.com/WGC_Shop/...ation.htm#agcd
http://www.airsoftplayers.com/manuals.asp
I'm sure they're more. If you find some good sites with gun diagrams, manuals or sckematics, post them in this thread!