So today I took out the regulator and made an adapter, making the tank, plenum and valve all one big plenum volume, about 32 cubic inches and set it back up, pumped up to only 2500 psi and pushed a 388 grain slug at 725fps! Much better but still not good enough.
I have a different idea to gain quite a bit more velocity and I'm gonna give it a try, maybe tomorrow. I'm hoping to push the 250 grainers at 1100 or more when it's all said and done.
If ya keep goin up in volume/psi, you're probably going to hafta adjust/increase the hammer spring, so ya get the full volume each shot.