There are two possibilities that I can see of, but you would need to get some more accurate measurements.
1. A Sharps 1876 to 1880's they are .450 diameter, and lengths of either 1.09 or 1.40 or 1.11. (weights of 509, 387, and 398 respectively)
2. .45 caliber Whiteworth lenght of 1.28 and a weight of 516
Hope that helps a bit...Lance