Like Casimer said, the cheaper spring mechanism guns usually require you to work the slide (or whatever else cocks the spring) for each shot.
My friend bought a Glock pistol that uses green gas. Was a very realistic feel, with green gas stored in the magazine so you could even to reloads if you had extra magazines.
Check out
http://www.kapowwe.com...I haven't been there in along time, but they used to have some pretty good you-tube video reviews of airsoft guns.