FWIW:
* H&A underhammer rifles have undersize bores, compared to other maker's - my .36 H&A Heritage took patched .340" balls, which I got from
www.trackofthewolf.com (I would strongly advise obtaining an extra nipple specific for the H&A, and a nipple wrench, from TOTW @ the same time)
(Other brand .36's take a .350" ball)
Alternatively, .330"
OO buckshot or a Hornady .314" SWC boolit can be used, both with patches.
* I used (and still do) about 15gr (+/-) FFFg for my H&A and T/C .36's - any brand of Holy Black you can get your paws on will do nicely.
* A longer ramrod (RR) that the issue striped RR will be needed for cleaning, and also make it easier to load - I made one about 10 years ago from a 48" long orange driveway marker, cut to length (I like one just enough longer than the bore for the fist of my hand to have enough room to work the RR), with boughten (TOTW) brass RR ends epoxied/pinned on. (I've had NO issues with rifling wear @ the muzzle from the RR)
* An inline capper makes things easier when reloading, and speedloaders (google both "for sale") are great when/if hunting with it.
They're very nice lightweight hunting rifles, and can really shine with a peepsight.
.