I have a couple QB's. They are undoubtedly the best bang for the buck in the airgun world right now, especially if you buy one of Mike Melicks tuned QB's...$100 for a $75 gun, tuned including shipping....what more is there to say? Not going to get one straight out of the box from China for much less $$ and the tune is worth what you pay for the gun, his work is amazing!
http://www.airgunartisans.com/flyingdragon/Haven't tried going the PCP/HPA route on one of these guns (yet), but I'm guessing it could be done. Mike mentioned on the forum here some time ago that you would need a regulated bottle(850psi or so, just like CO2) to run HPA in one of the QB variants, otherwise shot count sucks and the velocity will taper off with each shot. I think he's most likely right, HPA isn't in a liquid form like CO2 under pressure, so the volume would definately be less....sorta like a CO2 cartridge gun when the liquid is gone(i.e. the last couple shots). If you wanted to have a totally self contained HPA QB you'd most likely need some way of regulating the pressure to the valve from the air resevoir...maybe something inline? Huhm, now you have me thinking again...bad, bad, habit, I should watch more TV:)
So let me put this together...Bulked QB78, inline pressure regulator(something before the poppet valve, inside the tube), HPA..... I think the only thing we would need to work out is the inline regulator....might be cool if it was externally adjustable too.
Really like this line of thinking...
Dan