I've ordered a 1841 Mississippi in .54 cal from Jedidiah Trading. They're out of the office until 6/14 so I can't confirm that they do indeed have this rifle in .54 and what brand it is. Their website shows it under the Navy Arms brand but I don't think that's accurate. Could it be a left over Euroarms?
From my reading a .526" ball might be a better fit than a .530 due to the shallow rifling and the fact that I'd like to keep the patch thickness to at least .015" or .018". I can order .526" round balls from several places but I've had no luck scrounging up a .526" mould on the forums or eBay.
If they do, in fact, have a left over 1841 in .54 cal with the seven grove rifling and their webpage isn't simply out of date, are these repo's thought to be fairly accurate?
I had an Armi Sport Zouave in .58 that I used strictly with round balls but that rifle didn't make my "A" list during my last gun downsizing. I had good luck with it and it's shallow grooves, once I got a good patch/ball/powder combo worked out but a .58 can get to be a bit punishing if one desires a fairly flat trajectory. I know these aren't the type of arm normally talked about on this forum but I've never found such a collective heap of experience to draw from when shooting a patched round ball.