Then there's no reason you're idea wouldn't work just fine. Well, one reason, you'd end up with a pretty muzzle heavy rifle. I've considered some sort of twin tube arrangement, but I really don't want to give up the regulator, and I don't know enough (yet!) to make my own. So in the meantime, I'll keep using the HPA bottles.
As far as my new tube is concerned, I'm going to more or less duplicate the stock QB tube, just use a known material, probably 4130, with a thicker wall. The tubing I'll be using has almost the same ID, the stock valve will still fit inside,and a slightly larger OD. It'll still have the threaded cap, I need that to make the reversed tank block work, and that's potentially the weakest part of the design. But it will still be much stronger than the stock QB tube, so I'm confident that a 2K psi working pressure will still be quite conservative. At least, that's what my calculations are telling me, lets hope I didn't drop a decimal point somewhere
. The milling will be pretty basic, just some slots and holes, with the rest of it being lathe work. Since I'll be divorcing the cocking from the bolt by adding a separate cocking knob, I'll actually have fewer holes in the tube to deal with. Later.