Hey Longhorn, saw your post and signed up to reply and follow this thread.
I just bought a 712 g2 utility, and had the exact same issue. Don’t know why, but after cleaning it, then putting the barrel on for the first time it seized up just like you described. Never had a shotgun do that. I could not budge the barrel, so said F’it and found a crescent wrench and wrapped the handle in electrical tape. Then slid it between the barrel and mag tube, and grabbed my small brass hammer to persuade the barrel to come off. After several good whacks the barrel came free.
Could not see what was binding it up. Lubed the crap out of it and tried again, same thing, but not as jammed. Gave it one last time and got the barrel on, but was tighter than it should be. Barrel doesn’t look fully seated. Can see slight shiny metal ring above the black receiver. Only thing I can think of is the barrel is slightly out of spec where it engages with the receiver.
Hopefully someone comes along that has an answer for us both.
