When I was shooting my original M1860 Spencer carbine, I found the Lyman #533476AX (a little different, IIRC, from the mould without the suffix, but not much. Interesting, I shot smokeless powder loads in cases that were cut down DGW .50-70 brass. I cast the bullets using Lyman #2 equivalent alloy, rather than pure lead or 1:20 tin/lead. I got acceptable accuracy at 50 yds. My S&S centralfire breech block required some fitting, carefully using a file on the rear of the block. I would suggest that you slug the bore, but you may find the groove diameter at the breech to be wider than that at the muzzle! Yes, I believe the bore and groove on these were tapered! I may be able to give you more information, when I dig out my data.