Depending on where you live (USA/France/etc), felons can still buy cap and ball revolvers. I live in a state where you can buy up to 1 lb of black powder due to fire safety regulations. In Virginia, violent felons cannot own any antique firearm. My state can, and under federal jurisdiction I can. I'll take a stainless steel Pietta/Uberti model 1858 New Model Army percussion revolver w/ Goex FFFg black powder, a black powder measure, lubricated felt wads fitted for cylinders, .451 lead balls and no. #10 percussion caps w/ capper. You can buy any muzzleloader in the US, made in or before 1898 or after, provided that it can't be converted to fire fixed ammunition via conversions done to the barrel, bolt or breech block. Cap and ball revolvers only need a conversion cylinder to fire conventional centerfire or rimfire ammunition, however this does not exclude them from the definition of "antique" simply because cylinder is not included under the list of requirements for modification to exclude it (which would be, again, the barrel, bolt or breech block). One example is this double barrel muzzleloading shotgun that could easily be converted into a short-length double barrel 12 gauge shotgun. Another exception is modern firearms (say, an AR-15) converted into a muzzleloader. It can only be a historic gun, replica of a historic gun or just a regular muzzleloader, provided it cannot be readily converted to fire fixed ammunition by modification to the barrel, bolt or breech block. The next problem is that machine guns made before 1898 (like the gatling guns of the Civil War) are still considered "machine guns" per NFA definition, and may not be protected under "antique" status. You can even use cartridge-loading firearms, but only auctioned ones at very high prices, assuming you're lucky enough to find one and make a bid on it at, say, a gun show, but it can only be used for firing discontinued ammunition that is no longer commercially manufactured. Here are some examples:
>.41 Swiss — used in Swiss Vetterli rifles; essentially obsolete as a commercial cartridge.
>.43 Spanish — used in Remington Rolling Blocks and other 19th-century rifles; no normal commercial production today.
>.43 Egyptian — associated with Remington Rolling Blocks; obsolete.
>.50-70 Government — historically important, although ammunition has been produced as a specialty item, so it isn't necessarily a good example of something that is completely unavailable.
>.45-70 Government — definitely not obsolete for this purpose; commercial ammunition is readily available.
>.44-77 Sharps — obsolete/collector-only cartridge.
>.50-90 Sharps — obsolete in ordinary commercial terms, though specialty ammunition can exist.
>.40-70 Sharps Straight and various other Sharps proprietary cartridges.
>.43 Mauser — obsolete military cartridge.
>.450 Adams — obsolete revolver cartridge.
>.442 Webley — obsolete British revolver cartridge.
>.455 Webley — important caveat: obsolete as a current military cartridge, but commercial/specialty ammunition still exists, so it doesn't automatically meet the federal definition.
>Numerous early pin-fire and other unusual cartridges are even more clearly obsolete.
As for obtaining the ammunition? You're better off paying a gunsmith to manufacture the rounds for you. But either way, muzzleloading percussion revolvers are optimal in terms of accessibility. You only need to measure out 25 grains of FFFg black powder via powder measure and poured directly into each cylinder, then apply a felt wad, then place a .451 lead ball over it, and then rotate the cylinder with the ball on it to the bottom to then press it down to the bottom of each cylinder, removing the lead ring around the cylinder. Repeat this until you've loaded all 6 cylinders. Then apply no. #10 percussion cap to each nipple via capper. Then cock the hammer down to full cock before pulling the trigger to make each shot. Also, when concealed/open carrying, rest the hammer on an uncapped notch so it's not touching the primer while tucked away in your holster (in California, you can publicly carry these, however uncapped, although I will say even California is considerably more relaxed than Virginia is on this).
What do you carry where you live? The AEA Harpoon PCP airgun is another good option, depending on where you live.
