thanks for the responses everyone, the answers were a little more thorough than I needed. For the story, the mc's personality is not consistent with buying a firearm, so he owns one through inheritance, it being a family heirloom. I chose a pistol, and from that era, because it ties into a few other aspects of the story. I just wanted to make sure that these things, if being all original, still work reliably after 150 years
If the gun was well cared for, it would work. But, the character is going to have absolutely no idea how to load and fire it if he isn't into blackpowder shooting as a hobby. This involves loading each chamber separately with loose, pre-measured blackpowder, seating each bullet with the rammer, and applying grease or wadding to prevent chain fires.
Granted, if he's motivated, there is info online, including Youtube videos, etc., but it would require some effort for him to learn what he needs to know and than acquire some blackpowder, some projectiles, etc., to load and shoot the thing. Remember, the ammo for the period cartridge conversions is totally unavailable, so it's blackpowder or nothing.
If the gun doesn't absolutely have to be a Remington 1858, and you are willing to go just a few years later, the Colt Single Action Army debuted in 1873. The SAA was designed for metallic cartridges from the get go. Since the Colt SAA uses what is essentially modern cartridge ammunition it would be MUCH easier to shoot this pistol.
The original Colt SAA was designed for blackpowder loades (in metallic cartridges) but would be safe for lightly loaded modern loads like those used by "Cowboy Action Shooters." To get ammo he'd just have to go to a reasonably sized gun store and buy "CAS style" loads.
Now, if a 1873 revolver doesn't work with your story, that's cool. Just thought I'd mention it because it eliminates the problems of "no modern ammo available for the Rem 1858" and "He'd have to learn how to shoot front loaded blackpowder loads to shoot it."
http://en.wikipedia.org/wiki/Colt_Single_Action_Army