For brass there is Jamieson's and Bertram, although it is gawdawful expensive.
Magtech in Brazil makes 24-gauge brass shotgun shells, 2-1/2 inches in length. These can be annealed and sized down to a .45 in a full-length sizing die, then trimmed to 2.34 inches. The dealer for Magtech in Canada and US is supposed to be Cabela's and they do stock much of the Magtech line of brass shotgun cases.
If you're desperate, you can make up a die, get some Fiocchi plastic 24-bore shells (available as primed empties from the guy who imports Yildiz shoguns), lube them thoroughly with something like Lubriplate 105, dunk the front half in boiling water and then force into your die. It might take a couple of shots to get it right, but it will work.
Lee Precision makes dies for this number, but they are the big monster dies, so you will need their big press that handles the 1-1/8" dies.... or an RCBS Rockchucker or something else with that BIG hole.
MOST Martini-Henry bores are pretty 'generous', one might say. I measured one of mine a short while ago and the BORE measures .456, so a .457" bullet is NOT going to shoot worth a darn. Fortunately, Lee Precision also makes a mould for a HOLLOWBASE .459" slug and this is what I'm trying out next. BTW, Lee also makes a DANDY lube they call 'Liquid Alox'. Very easy to use and it works very well. My 1871 Remington likes it.
You should know that original English-made Kynoch cartridges had an INSERT inside the brass case, from the base up to the shoulder, about 1/16 of an inch thick, made of cardboard and lacquered. The purpose of this insert was to REDUCE the internal capacity of the casing to approximate the original Boxer wrapped-foil cartridges. In this way they could arrive at the light compression which Black powder requires WITHOUT going to a charge of about 120 grains. The Martini likely could handle this kind of charge but it is entirely likely that your shoulder could not. If you end up with casings which do not have an insert of this type, be sure you add a wad of some kind to keep the powder solidly in its place against the primer. Just one of those silly drugstore 'cotton balls' (which aren't balls and are not cotton) should do the job.
Your correct charge is 85 grains of Black powder with a 480-grain bullet. I would think you could get away with FFg or FFFg: the original RFG Number 2 was about halfway between these.
If you want your brass to last MUCH longer and you don't like pouring endless quantities of boiling water down your barrel, you can always load this critter with the old-time standby, SR-4759 powder. This is ALMOST a Bulk powder, but not quite: there are NO true Bulk powders still in production. This is a very bulkY powder which you can use at 38-percent-of-Black-by-weight and get good, consistent igntion, excellent accuracy and a cean barrel with. For your rifle, the charge would be THIRTY-TWO-POINT-THREE grains by WEIGHT, NO MORE. This should give Black-powder performance and pressures both in your rifle. It should be available anywhere that stocks the IMR line of powders. The designation 'SR' stands for SPORTING RIFLE. This powder replaced the old DuPont Bulk in 1941 and has been out f production more times than you can count, but it always keeps coming back because it is NEEDED for rifles such as yours.
Hope some of this helps.
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