I really like the semi-coaxial style, how you used the space around the barrel for additional chamber volume.
How did you seal the barrel into the larger pipe? Epoxy, JBweld, something crazy with fittings?
Did you mod the blowgun? I assume so, though I doubt it matters for piloting a 1/4" QEV (You could probably pilot that valve with a pinhead...).
What's the lowest-rated part? The hose, I assume?
I bet the performance is great with those teeny ball bearings. I'd try making some darts for it (though you'd have to muzzle load them).
The hose at the bottom is the only part I don't like. It'd be nice if you could figure out a way around that, but I see the problem: those fittings don't line up well enough to use a nipple there.
I'd consider trying to bend a fitting or pipe (maybe using copper, it's easier to bend...) or using JBWeld to make a little angle adapter (I've used JBWeld in some pretty high-pressure, high-stress applications before and have always been impressed with the results).
Since it looks kind of like an SMG already, a mock silencer (that'd fit over the last few inches of the barrel, making the whole barrel look much bigger) and a shoulder strap would be cool accessories
Nice work.
Peace,
Pete Zaria.
