Converting a Martini-Henry will require, just for a start, a new barrel and a new extractor. These days, that's about 400 bucks' worth of conversion, plus a refinishing job..... and you end up with a destroyed antique.
It would make more sense to spend about half of that money and pick up a box of brass, a set of dies from Lee Factory Sales and a Lee .459-405 hollowbase bullet-mould. The slug is a bit light, but the mould is only about 25 bucks and the rifle likes the slug most of the time.
Forty-five-seventy might be a famous American cartridge, but a point to remember: the Martini-Henry was the most powerful black-powder single-shot military rifle in the world. Rather than just a .45-70-350 (Carbine load) or a .45-70-405 (rifle load), the Martini was an honest-to-Gawd .45-85-480..... although absolute utter wimps such as myself definitely download it. The expression "kick like a Martini" is MUCH older than the drink!
As to converting a SNIDER....... Why? Better to get some dies (Lee) and a bunch of 24-gauge shotgun brass (Magtech out of Brazil; Cabela's is the importer) and a .578 Minie mould (Lee again) and go have fun. MUCH cheaper..... and the rest of us don't have to duck. I am not at all sure about running anything in a Snider action other than .577 Service-spec ammunition..... or something with an even lighter loading. Remember, most Sniders had soft IRON actions: only the Mark III had STEEL at all!!!!! I do NOT want to be on a range if a Snider is shooting anything except .577 or .297/.230 Morris. I most emphatically recommend AGAINST this conversion.
Besides, original Sniders are now 150 years old or very darned close to it. They deserve a little respect; after all, they were the FIRST.
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