Try and buy a REAL Parker-Hale version - serial numbers anything up to 9000. There are a good few of them around, like mine, serial #1187 [not for sale].
Make sure you have to correct bullet mould - the Lyman deep-cavity version is the way to go. The bore needs measuring so that you can get a fit that is 0.001-0.002 smaller than the muzzle.
The bullet has a deep concavity that maximises the blow-out effect in the chamber where the rifling is intentionally a lot deeper than it is at the muzzle. Later guns, made entirely in Italy by EuroArms, do NOT have this prototypical feature. Guns with serial numbers between 9001 and 14000 have P-H barrels, but little else. Check the stamps for Birmingham Proof House and proof load of 90gr.
The service load is 2.5 drams - 68.5gr of FFg. Lots of them, mine excluded, shoot quite well with loads as low as 45gr. A good stiff lube is needed, too, I make mine from 60/40 beeswax and olive oil.
When you come to replace the nipple - they EAT nipples - make sure to get the copper spacer washer from Peter Dyson - their use makes removing the thing a lot easier. The very first sign of hammer blow-back - STOP right there.
They are great fun, but are nevertheless a very serious piece of military hardware, used mainly by artillery and dragoons as a kind of last-ditch defence. A friend of mine in Maine has taken everything allowable with his Musketoon, but being rather large, he has no real problems shooting a 600gr solid and 100gr loads. He has dropped a 3/4 ton buff in Montana on three occasions, too. Back in the woods, hHis hog shots enter the chestplate, and often come out above the tail. Here in UK, of course, it is not permitted to shoot live game with one, as they 'fail' the muzzle energy test.
Yeah, right.