Hi Deacon
Echo . . Echo . . Echo . .
PLUS ONE for 45 Dragoon. "If it's Mattel it's Swell". OK. Pietta does NOT make any cartridge conversions. PERIOD. Ergo: It's a Uberti. If it's a Uberti, it has an abysmal Barrel to Arbor fit. You Need/Must/Should fix that FIRST. Abilene (Bless Him) is incorrect. You may well get away with something, but that doesn't make the gun "right".
Normally, you cannot do the "turn it 90 degrees" test out of the box. Uberti hides the Abysmal fit problem by poor fitment of the Arbor to the Lug bore. The barrel just "grinds" to a stop. The Lug bore need de-burring and the Arbor end needs de-burring and polishing. THEN you can put it on the arbor 90 degrees out and turn it down.
There are several ways to "fix" the fit. Some require some machine work. I worked out a method about 20 years ago that uses a simple #10 stainless split washer down the lug bore, then stuff the arbor in on top it. Just don't forget it's there when cleaning the gun. A single split washer normally corrects 99% of Arbors. It's also some cheap. About .06
Play Safe out There
PS: Forgot to mention, once you get the Barrel to Arbor fit fix'd, you will probably need to re-fit the Wedge. And then replace the OEM springs.
PSS: You didn't mention if your "other" open top pattern gun was a conversion or percussion. If it's percussion and Pietta your Barrel to Arbor fit is so close you don't ant to mess with it. If it's a Uberti . . . . Trust Us . . . . The Abysmal barrel to arbor fit just hasn't reared it's ugly head (yet)
People are Still Hazardous to yer Health