+1 to whats been said
BEST way = go to a local club where folk USE BP, then try some of what they have & see what suits you
I went the route of the 1858 over the Colt type...couldn't hit a barn door with it though
Then, saw a nice .36 Colt Navy - sold the 1858 & got the Navy - magic bit of kit (then found out that ALL the prize winners in the club were using .36 Navy's!!)
Now using it with paper cartridges which means I can weigh out the powder at home (as I gave my flasks away) - plus the original guns were issued with paper cartridges, so more "spirit of the original" - besides, its much less mess & quicker to do on the day
For rifle, also look at the .451 types - Whitworth, etc.
As has been said - a smooth bore with a bbl over 24" is just classed as a shotgun so will be easy to buy/try
Good luck - enjoy the black arts
.22 BRNO Mod 2 & 511, .22 SGC V-22, .223 (Armalon), .30-06 M1903A4 & Win 70, .36 Colt Navy 1861, .45 Win 94, .455 New Service. 410 Webley b/a, 12ga: Ithaca 37DSPS & FN A5 & Rem 870 & Win 1897, 10ga "Greylag"