I've started working on another QB 78 setup to run on HPA.... The twist on this one is that the tank is reversed and will act as the forestock.... I did this on my Hayabusa project, and the QB 78 main tube was just the right length.... I'm using a 13 CI Ninja 3000 psi tank regulated to about 1600 psi and equipped with a 1.8K burst disc.... The tank block is machined from a block of 1" thick 6061-T6 aluminum that measures 1.37" x 2.63".... It is wide enough for a 3-groove Picatinny rail on the bottom for mounting a bipod and/or sling.... The tank is tucked up against the bottom of the main tube with minimal clearance.... The assembly attaches to the front of the main tube using the stock knurled nut with the hole enlarged to 1/2".... That is the only modification to the gun necessary to attach the tank, other than shortening the stock....
There is a brass fitting with O-ring that fits into the front of the QB main tube and is retained by the knurled nut.... The O-ring is moved to behind the threaded part of the tube for additional strength.... That fitting is attached to the tank block using a steel 1/8" NPT hex pipe nipple, with the knurled nut captured in between and free to rotate.... Below where the nipple screws into the block I machined a standard paintball fitting that the regulator output of the tank screws into.... There is a pin that presses on the pin valve in the regulator to open it.... The air passes around that pin and travels vertically through the block via a 1/8" hole to where the pipe nipple is.... and then from there into the main tube.... The bottom of the 1/8" hole is plugged by a 1/4" long 10-32 low profile SHCS recessed flush with the grooves in the Picatinny rail, and sealed by an O-ring....
Also in the photo above is the highly moddified QB valve.... Basically, once you open up the front of the valve, all it does is provide a place for the spring to sit.... With that in mind, I machined away all the extra material, so the valve is now the entire volume of the main tube, about 50cc including the valve area.... This is a large enough volume of regulated air to support up to 50 FPE shots without losing efficiency.... Since the eventual plan is for this gun to be converted to .25 cal, that is ideal for my purposes....
Bob