Back in the day, I was able to run my 68-Special on a remote, with a siphon.
The first 10-15 shots had low FPS, but once the liquid gets into the valve, its all gravy after that, and it ran pretty well.
A few have talked about ways to upgrade the Tippmann valve to run HPA. The problem is not springs, or hammer weight, but the valve volume. With liquid guns, the valve volume is made as small as possible to improve liquid effeciency. With HPA, the valve acts as a wicked bottleneck, and it won't flow fast enough to push a paintball out at 300fps.
A solution would be to drill into the side of the valve, through the body, and add an LPC. Enough slack so the valve can still float freely. Thats alot of work though, for no actual benefit beyond conveniance
EDIT- Also, I think Azzy talked about using an HPA tank, modified to output at 1400psi? or higher? Not sure how that worked out though. Seems to me that after a certain point, the pressure is going to hold the cupseal closed, and you may never get the FPS you want with that method
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