CS, would you mind posting the McMaster links for Sch. 80 CPVC and the aluminum tubing. I haven't had any luck finding them on McMaster.
EDIT: I found the Aluminum tubing, but am still looking for the Sch. 80 CPVC link/part number.
Is this is?:
Macmaster part #6803K52
And Lynx, I'd go with somewhere around 4.5". Maybe a little more, maybe a little less. Experiment with it a bit, find the perfect barrel length for maximum range.