Pulled out the AR with pliers and Dremeled it down to a hole, then decided to do a CPVC barrel-replacement and just shaved the original "barrel" off where it meets the plunger and Dremeled out the plunger front until a ~2" piece of 0.5" CPVC could squeeze inside, back end level with the plunger head and front end level with the trailing edge of the magazine (which I Dremeled out to accommodate the slightly-longer streamlines). Sharpened the leading edge of the CPVC, just like the original barrel, and cut a notch at the top to help in dart separation.
Original Barrel:

CPVC Barrel:

After all this, I'm still getting flat ranges in the low 30's, almost the same as having ARs in and original barrel materials - it'll load standards and streamlines equally well, but just kinda pop them out the end, no real oomph. I know it should do better, 'cause I originally had a 6-inch barrel attached to test the material's suitability, and it practically put a streamline through the wall (which makes singling it very tempting, but I want to try and keep the magazine function, if at all feasible).
Any advice? If you need a picture of the whole gun, check out this thread: Clip Blaster Internals