Look at English pocket pistols ca 1820 (online auction sites, etc.), and the few Hawken and related pistols which are very similar. Most have a swamped or tapered barrel of large bore, the ramrod is near bore size at muzzle with a rapid taper to rear. With a .54, start with a good 1/2" rod at muzzle, maybe a 7/16" forward pipe and 3/8" entry pipe, and taper the rod smaller inside forearm - with or without an iron tip for jag inside. The muzzle end is sometimes sleeved with bone, and if plain, is usually rounded out at end, versus concave to fit ball.
These online auction sites like Guns International are great resources for studying old guns. Many other details can be picked up and approximated - key stock for front sight, furniture, finishes, etc.