[DeadParrot]

Most modern in-line hunting muzzle loaders are fitted with a nice optical scope. No reason a 1800's era rifle wouldn't benefit from a modern passive optical scope. An 1800's pistol might well be more useful with a red-dot type sight.

Once you have the mechanical mounts in place, price would be the limiting factor on what optics(passive or active) a shooter/sniper could install.

For many applications, a nice night scope would be a big upgrade for a sniper in such a world. The single shot limitations of a muzzleloader might not be much of a limitation in a shoot then vanish into the night scenario. Especially if a night scope allowed a 200~300 yard or longer shot.