HiPacs are limited in performance (for and given pressure) because the only air "on tap" and available to make the shot is the air inside the valve because of the small hole between the HiPac and the valve....Bob
Here's the one I did for a 12fpe pistol.http://www.gatewaytoairguns.org/GTA/index.php?topic=61428.0
OK, so you want to build a PCP.... so yes, the idea of using a 2240 or 2250 tube is useless as they don't have enough volume.... That leaves you with a HiPac, which to get high power will require running very high pressure, and you will have to live with a heavy trigger pull.... OR you can run a 2260 or Disco tube, change the valve to one that seals in the tube and mounts properly, and install a Disco (or B&A) fill fitting in the front.... If you want to run 2500-3000 psi you will have to use better screws than the Disco uses, they are not designed for that much pressure.... and you should probably have the valve seal in front of the roll-stamp if you choose a 2260 tube.... I don't know how many HiPacs and extensions you can screw together, nor the air volume and weight, but considering the limitations the design put on FPE I would go the 2260/Disco tube route.... I think you can buy custom length Disco tubes (from B&A?), use a Disco or B&A Boss Disco Valve, and then address the stronger screws to handle the pressure you want.... Then once you do that, you will have to find a hammer spring that will handle the pressure, and live with the trigger pull it generates.... Changing a 22XX or Disco to run 2500-3000 psi isn't just a simple thing to do, and every change requires two more.... and those two, four more.... etc. etc.... so be prepared to chase it for a while until you figure it all out....Bob