How do I build an airsoft gun?

             I want to play airsoft, but all of the guns that are relatively cheap seem pretty crappy, and all of the good guns are really expensive.  Thus, I want to build my own AEG with the following specifications:  Full/semi auto capability; hop-up; and 100-200 foot range.  I saw the basic inner workings of one in the comments on an airsoft instructable, and it looks like something I could do.  Any help with machining ideas, parts suppliers, and detailed plans are welcome.  Thanks!

http://www.burntlatke.com/bb.html
and
https://www.instructables.com/id/BB-Machine-GunNOT-AIRSOFT!!!!!/

are both very good places to start looking at for a fully auto gun.

also look at www.spudfiles.com
that is the bible for all this kind of stuff