Of course, the first step is getting the durn things out. One thing to remember about Uberti. Uberti has a pet Gorilla (his name is Bongo) who wanders around the assembly area at night ....... Snugging down anything with threads. Uberti also states they DO NOT use air tools
on their assembly lines. Ok. Sure.
Pettifoger is correct, once you get Uberti threaded thing loose, they are just fine. When you do get the nipples out, give the nipples and the receptacle a real close inspection for rust. Uberti often includes rust as a "no Cost" bonus with their products.
As has also been mentioned, most "T" handle nipple wrenches are at best a swell paper weight. Once you get the nipples out, they can be quite serviceable. First, though, with a small file, cut the bottom of the notch in the wrench flat to match the flats on the nipples. I use the ratcheting wrenches from the Possibles shop for stubborn nipples. For routine maintenance, I like the "cross" wrench shown at the bottom of their page. The screwdriver end however, is useless and it will hurt your hand using the other end. Cut it off and dress it smooth.
Heat. Heat can work wonders for stubborn threaded things. Especially if some dumb rube has used "Red" LocTite. A very handy thing in the shop is a small hand held micro torch. Clamp the offending cylinder in a padded vice, heat the area around the offending nipple and then go for it.
Incidentally, I alway contribue to Uberti's annual "Bananas for Bongo Drive"
Coffinmaker
PS: Almost forgot. Permatex Anti-Seize lubricant. Then just finger tight the nipples. Torque on the nipples is NOT your friend.