i have no doubt whatsoever that removing the bolt spring would give you a noteable jump in velocity, an extra 30 or 40 fps wouldnt suprise me at all. not only would the bolt reach a much higher velocity but without the spring to return the bolt you get a complete dump from the chamber wich is in the neigborhood of an extra 50 psi(this isnt something that im willing to verify, if i hurt my mag it costs time and/or money to fix it, and that would suck).
how rapidly a gas expands doesnt change the total amount of cooling, only how steep the temperature spike is.
the ball should encounter the same barrel friction reguardless of what you use to push the ball.
im thinking that drilling out the center of the bolt plunger and filling it with epoxy or something along those lines might be a good plan to further reduce the weight of the bolt...hmmm... a titanium super bolt with an epoxy filled plunger...sounds pricey
If your body is really wierd, try showing it to people in the streets for money.-Hitchhikers Guide to the Galaxy, Tips for aliens in New York, Surviving