The Bull Dog was a .45 Webley variant of the 1870's (allegedly carried by Custer at the LBH) so if you're looking for something earlier, a Tranter or a Beaumont-Adams seem the most likely candidates from the 1850/60's: the latter was a 5-round, .442 cal (54 bore), which would fit closely with your recollection
but I'm sure there may be other, better informed, suggestions to come...