Another way to tarnish up the brass to get an 'antique' appearance...if that is important to you, is to simply shoot bp cartridges for a period of time. The bp residue dulls and darkens the shiny metal, in no time. Of course, the downside of this method is having to properly clean the firearm, after use. Not difficult, but does require some care.
Modern reproductions chambered in .44 WCF (.44-40)...like JohnnyPython's beauty(I prefer the 'iron' frame, too), being some of the easiest to maintain. As their slightly bottle- necked cartridge cases are quite efficient at preventing excess blowback into the receiver. On the plus side, a shooter gets to experience what it was really like to shoot these rifles.
A bit off topic, but...one of the most satisfying rifles of the era that I ever had the opportunity to shoot, was an original, 1880-vintage Winchester 1873 rifle, chambered in .38-40. The elderly gentleman who owned this fine shooting specimen, had inherited it from his grandfather. Who purchased the rifle new, in 1880. True to it's design, the rifle had always fired bp cartridges. Even up to the time I shot it....in the mid 1980's. I should add that the owner was an avid handloader, who knew bp cartridges, inside and out.
Over the years, the rifle had piled up an impressive list of game to its credit. Well kept and in beautiful condition.... despite the years and much use, it was still just as accurate and smooth operating as it had been, new. It was a pure joy to shoot this fine old rifle.
Al