|Winchester 1894 (Source)|
I recently posted what I thought were the 5 greatest myths of firearms among preppers. I listed among these the advice from some circles of stashing away a black powder firearm with the idea that post-Apocalypse, one would be able to manufacture black powder. I noted that black powder actually requires some form of civilization and trade to manufacture and obtain supplies, as well as issues with procuring priming materials (flints or percussion caps) and lead for making bullets.
I suppose, though, that some may hold contrary views. In that regard, I would suggest a better black powder weapon than a muzzle loader--the black powder lever action rifle and large caliber revolvers. There was a substantial overlap between the invention of metallic cartridges (and reliable firearms to shoot them) and the adoption of smokeless powder. Some of the most famous rifle and pistol cartridges originated during this period, and some survived to be used with smokeless powder.
There are a couple considerations on selecting a dual purpose cartridge--one that can be loaded with black powder or modern powders. First, black powder does not develop the velocities of modern rounds. Rifles and handguns compensated by shooting a large, heavy lead bullet. For instance, .50 caliber or larger was common prior to the introduction of metallic cartridges, and many of what we would consider effective cartridges were at or near .50 caliber. While there were smaller calibers offered in black powder, they would be considered very anemic by modern standards. Thus, our survival arm--intended for hunting or self-defense--should be a large caliber.
Second, I would not recommend buying and using a firearm from the 19th Century for survival purposes. Fortunately, some firearms are still available that shoot these old calibers. Two that come specifically to mind are the .45-70 which is still manufactured in lever action rifles, and the .45 Colt (sometimes called the .45 Long Colt) which has made quite the comeback as a pistol cartridge due to the popularity of cowboy action shooting and the Judge line of revolvers from Taurus. I'm not saying that other cartridges might not work, but these are far more common than other black powder compatible cartridges, and information can be found for reloading both.