I had the exact same problem where my Mossberg 930 JM's barrel would not slide back over the tube.
I checked, double checked, triple checked documentation and videos online. All the videos online seem to show this is an easy process but I literally had to wiggle and jam that thing back on hard!
When I get home, I'm going to check if my barrel is canted too...
Take it outside and see if there still is a slight veer in the barrel. indoor lighting has a tendacy to create some weird shadows on vent ribs making the barrel look "bent"
Looked this thing over every which way, and it definately veers to the right. I took it apart for it's first cleaning and to check it over. Went to reassemble it and found it almost impossible to get the barrel lug back over the gas tube. I honestly thought for a while I would be sending it back to the factory in parts. Finally after wiggling turning and pulling every way imaginable it slid back over the tube and I was able to complete reassembly. I can't believe all the 930 barrels slide back on that hard. Called Mossberg customer service, told him the problems, and got a shipping label being emailed to me next week. Frustrating, but I guess worse things can happen. I'll try to remain optomistic that Mossberg will take care of this.