Well Hi there Im2bent.
Welcome to the joys of Uberti Barrel to Arbor fit problems. the Arbor should be pretty close to a uniform diameter. Most Uberti aren't. You're not experiencing anything "un-normal" for a Uberti.
First step, make a "strap" of 220 Wet/Dry. Clamp the frame in a padded vice. Use the strap as polishing a shoe. Keep turning the frame in the vice so you polish evenly. Also make a Flap Sander out of a chunk of dowel. You will want to wind up with a folded flap of 220 Wet/dry. Your first strap should be about an inch wide, the length of the paper stock. Your flap sander should be a length of paper the same as your first flap, folded in half. Make the slot in your dowel the depth the folded piece is wide. Cut to length so your stick is completely wrapped. Chuck it u inna drill and run it into the Arbor Hole in the back of the barrel lug. Run the flap sander in and out to eliminate any burrs. Then polish with 800 grit. As you work, try fit FREQUENTLY. Getting a Uberti to set up is NOT fun.