1860 barrel replacement
Kisrt, the maker of drop-in cartridge converters points out that the Pieta army cylinders are a little longer than Uberti cylinders and, therefore, Kirst recommends their .45 Russion caliber conversions for Uberti revolvers instead of .45 Colt because of the difference in length. You might be trying to fit a Uberti barrel instead of a PIeta.