For All You (model) Gun-Nuts, You Want A Gun?! I'll Give You A Gun!!!
Actually, its not K'nex, its Lego-ah-wait a sec! These guns are uber real and eject shells (yes, they actually fire [poorly *25 ft*]) Although the instructions are sparse, if you look for them (e.g. on the aforementioned Thomas Cooper page) you will find them, and boy is it worth it.
Without further ado, the link (actually TC's page)
WARNING! THESE GUNS TAKE A LOT OF LEGOS, so be prepared for a long build and (if you dont have a lot, lot of Legos [yes, that was intended]) a lot of scrounging for pieces.
So, I hope you enjoy these guns, but dont get mad at me if the instructions aren't extremely clear (I'm not even a member of MOC pages)
(sorry, no pics)