To fully advocate a cap and ball revolver for self-defense. I can only recommend the use of either a Ruger Old Army, or a steel frame Remington 1858 in .44 cal. I have shot all forms of percussion pistols, and found for total reliability, these two will meet the demands. Use a good load, I strongly recommend 3F GOEX as the propellent for self-defense. Quality caps, 32 gr or so of GOEX 3F, an over powder "wonder wad", and a hornady round ball, and you are armed. I don't use a topping of grease on top the cylinder for self-defense, I prefer to keep the revolver clean and don't risk residue running into the holster, or on my hands when I need a good grip at a moments notice. I do advocate grease, crisco, especially "wonder lube" for range use and extended shooting sessions.
Also, the Ruger and 1858 designs offer a fairly rapid reload with a spare cylinder, much easier accomplished than with a Colt replica. As far a power, I don't worry about it. I have shot enough critters with a .44 cal round ball to know it is a lethal pill at modest range. Lead has a way of doing remarkable damage on impact. I have read all sorts of balistic data, trying to assess the true power of the round ball. I enjoy and trust all the fine work by Sam Fadala, an expert on such matters. Look for his books on BP. My own field use, is that at modest ranges, anything a .38 Special, or 9mm can do, a quality .44 cap and ball revolver in good hands, can achieve the same. Some give the .44 round ball at about 900 fps, a near .45 ACP power factor. I don't go that far. Maybe with a good conical bullet, but I have not found a conical that allowed me at least 30 grains of 3F powder, and a wonder-wad for a load chain. I always use wonder-wads between the powder and projectile, to prevent chain fires. I believe you need at least 30 gr of 3F with the .44 cal to consider it worthy of self-defense. So from my experimentation, these are my own self imposed boundaries to even afford my own confidence in a cap and ball revolver. I also have a Colt Pocket Police in .36 caliber I highly tuned and modified into a snub-nose for concealed carry. It is less powerful, slower to reload, and by design I carry it with the hammer on an empty chamber, so I have only four rounds available. It is though a good back-up gun, especially to my Remington 1858. The little snub-nose is what I would grade a belly gun, and best used at arms length or a "contact" shot. I carry modern pistols generally for concealed carry, but every once in a while, I carry cap and ball pistols. The older I get, the more experience I have with them, the more confident I become. Used within there limits, they will work. I want to encourage anyone that hasn't tried a BP shotgun, to give them a try! If there is any BP firearm that can give it's modern form a run for it's money shot to shot, it is indeed the BP shotgun. You can devise some great load configurations that achieve all the velocity, power, and pattern considerations you desire. For self-defense, a double barrel .12 gauge BP "coach gun" is formidable. Yes, all BP firearms are slower to reload. But considering the true threat you may face in a confrontation, and gaining confidence in their performance, BP firearms are up to all but the most challenging tasks. Good shooting! tdv