here's a pic of a couple .43 Ive made up. the left is a paper patched to 446 backed by 4198.
no go. bullet is to hard, and remember smokeless is a progressive burn. wont deform the bullet enough.
the other is a .451 300gr pistol bullet. wont chamber.
tough. Ive loaded for trapdoors and some old levers but this is tough.
im getting a land to land of .434, grove is over the 451 slug i ran thru.
mine wont chamber a bullet much over .446.
OK with smokeless you need a bullet that is min. grove diameter. id say in the early '71's is made deep (451+) to 'hold fouling'.. they had an issue then with accuracy.
the twist is real shallow. like 1/21 or some such. short bullets wont work well IMHO. ive tried some track of the wolf 'mine' bullets but not long enough.
uboat534's method works. OK issues. you ll need DEAD SOFT lead bullets. progressive patched rear to front, and as much BP as you can cram into the new thicker brass. im not going there yet. Some folks with trim up to 1/4 off the top of the brass to get the paper patched bullet to chamber. each rifle is different.
i had an issue with getting even the standard BP load into the new brass. Ive had this issue with other BP cartridge rifles. the modern brass is way thicker than the old stuff.
there's a company , 'mountain molds' that will make a mold to suit. im not sure what i want yet, and ive got some other projects in the works. im pretty sure it'll be a 'minie' type, to keep the length.
not sure of the shape yet.
its one of the harder rifles Ive owned to get working. as with all my rifles i try something then think on it. LOL.
i think midway has a paper patch template. but again Ive tried it with smokeless and it ain't working.
ill get it going. im going to shoot up the rest of these bullets at gong targets and start fresh. LOL.
Try some 4198 say 30 grains and work up. real powder puff loads. i used the .446 bullet.
DISCLAMER this post is my OPINION, and i take no responsibility for anyone's home loading of weapons. the 1871 Mauser has NO gas check and a split case can result in a dangerous condition.
use this information AT YOU OWN RISK.