I had made a prototype based on the idea of combining a pressurised magazine with a detent " valveless pop-off " style breech:

http://www.spudfiles.com/forums/files/tubemag_165.gif

I was less than successful in practice and didn't feed reliably, though I blame it on my own construction of the ...

Some Feltman carnival automatic BB guns feed from a pressurized magazine, and so do classic "bullet valve" airsoft guns.

How much pressure can those fuel lines take?

Why would it add power? How did you calculate .01% additional power?

lol im just saying due to the fact that the pressurized air ...

If you want full auto without having to have a spring in the mag or having them fly back up the mag, you can attach a small rubber pipe (fuel line from motorbikes) from a valve output (the one that ALSO forces BB's down the barrel) and run it to the top of a magazine. On the opening of this valve BB ...