My thoughts on this are simply speculation at this time as I have not constructed my own afterburner rayven setup, but I've had this opinion on this setup since seeing it first implemented about a year ago. The inherent fault in this design is the length of barrel between the front and rear flywheel sets. In an afterburner setup, the rear flywheels should be serving a purpose soley to accelerate the dart to the extent that it is entering the front flywheels at a faster speed than initially at rest. Since the barrel section (faux barrel in your case) is longer than 1 dart length +/- ~1/2 inch, you can run into the scenario where darts are not propelled forward in a perfectly straight fashion and will then lodge in the faux barrel section (due to not hitting the front flywheels in a manner consistent with possible feeding into the flywheel cage).
I think you're right, although from the description of his problem doesn't seem to indicate it was getting stuck between the 2 sets of flywheels. That said, if it's not hitting the afterburner centered, it MIGHT increase the odds of exiting at a drastic angle that causes the dart to hit stuff some of that orange chud on the muzzle. Without having the blaster in front of me, it's tricky to precisely diagnose.
It's ~17mm. My calipers are being a bit janky recently so that's as close as I can get.
I would suggest a narrower tube nested inside the faux-barrel, but that tube needs to be slippery and possibly have a flared entry. I'd use http://www.mcmaster....8628k57/=suwkab
but I have to recognize that normal people aren't going to have that material lying around, and I don't have a hardware store alternative.