don't they come preported? maybe check and be sure all the o rings are good on the barrel transfer port and pellet probe.. unless you are shooting slugs that are too tight.. in that case I'd try regular and heavy pellets.. if your speed is good then you might need to try a different slug..
just thoughts off the top of my head..
been asked to machine new barrels for the avenger, but they didn't want to ship the gun.. and I always hand fit my machine work.. besides I have machines before digital readings were even available but they work well for me.. and like a rebarrel I'd use the receiver to make the barrel set screws and the machines ledge on the new barrel to get everything tight and exact.. so I just said that if it is not worth shipping across one state line for a price of just retired hobby work.. it's not worth my time.. I don't want to do anything that makes people not completely happy..
they were wanting to rebarrel with a lothar Walther barrel blank, and longer and wanted to make their own moderator adapter and crown.. and I don't want to take a chance that it's not perfect.. I can recrown in the lathe much better and quicker then monkey around with a Dremel by hand.. the Dremel is likely by hand to not be concentric and then who knows where it is going to hit..
if all else fails with your rifle and you are comfortable pulling the barrel you should check by pushing a pellet through it to see if you have tight spots or if it's choked too tight.. especially if you rule out everything else. the two needing a new barrel had either the barrel set screws drilled to deep or tighten too much.. or both because they had indents protrusion into the bore.. and I don't know if it was that way factory or I suspect someone used tools that they should have not touched.. and overtightened them? but it's a guess not seeing it in person..
T patner up this thread will know for sure if different size ports.. he's a great guy.
Mark