Black powder cartridges were common stay until ~1890s and some (e.g., .45-70) were very powerful. You also technically don't even need brass: shotgun shells use plastics and (prior to that) used to use cardboard. Shot/slugs/bullets... are all easy to manufacture as well (you just need a mold and lead).
The difficult parts are:
1) Getting the mercury fulminate percussion cap. I doubt this is something that can be easily manufactured. Even if you use a readily bought one (which, I'd imagine, could easily be tracked and/or prohibited by government), it would be quite dangerous to monkey-patch it onto self-manufactured brass.
2) Non-standard brass will not feed reliably from a magazine, even if it will chamber: so any such firearm will be limited to single shot.
3) You'll still be limited to low pressure rounds: I highly doubt a home made action would withstand even a strong black-powder round like a .45-70. I don't imagine this handling anything more powerful than an old .32 S&W or a .410 shot shell. Still dangerous, but not exactly a major caliber.
The difficult parts are:
1) Getting the mercury fulminate percussion cap. I doubt this is something that can be easily manufactured. Even if you use a readily bought one (which, I'd imagine, could easily be tracked and/or prohibited by government), it would be quite dangerous to monkey-patch it onto self-manufactured brass.
2) Non-standard brass will not feed reliably from a magazine, even if it will chamber: so any such firearm will be limited to single shot.
3) You'll still be limited to low pressure rounds: I highly doubt a home made action would withstand even a strong black-powder round like a .45-70. I don't imagine this handling anything more powerful than an old .32 S&W or a .410 shot shell. Still dangerous, but not exactly a major caliber.