Calipered measuring of the throats on my newly acquired 25-5 is at .454/.455. Mine is an N prefix pinned barrel that checks back to 1980. The cylinder has been cut for clips and I've shot some .45 Colt lead cowboy loads which seemed to group the best. Followed by some .45ACP 230FMJ with the 230LRN being the least accurate.
All firing was off-hand at app. 15 yards. None of which performed outstanding (could've been me) but the groups were acceptable for self defense shooting at nominal distance, although the point of impact varied quite a bit between the various loads.