There is another alternative. The guys on classicairsoft.net have been using the Monster Airsoft Workshop electropneumatic mechbox. Essentially, it's both battery-powered and gas-powered. The battery is used to power the gears with a stock spring - this controls ROF and feeds the bb's. There is no piston, and gas is used as the propellant. Obviously, it's still capable of full-auto. However, the price ($750 or so USD) for what is essentially a stock gun TM with gas conversion is too steep for many people.
Currently it's only available for V2 and V3 mechboxes, but he did also build a TM M14 capable of 550 fps @ 135 psi.
Quote from actual user:
My initial thoughts were that this rifle was worth every damn penny I spent on it and the thought that I would rather spend the initial high buy in price for this rifle instead of spending the time and money on an AEG. The other nice initial thought I had was that it would be nice to be able to go to any game and fall within their FPS limits with just a simple regulator adjustment. However, this rifle did have one tiny point that is unavoidable. There is no recoil like the recoil you would feel on an Escort tuned MP5 or M series weapon. However, due to this gun's unique nature, I am willing to give that up.
Video of Monster SR-25 @ 100/130 feet:
http://www.youtube.com/watch?v=sTKpVuTQKnk
Full Auto example vid from owner (cellphone vid):
http://www.classicairsoft.net/MOVIES/MGCAUTO.AVI
Reviews and discussion:
http://www.classicairsoft.net/forum1...?TOPIC_ID=6864
Monster Gas Cylinder blog:
http://monsterairsoft.blogspot.com/