Billinghurst noticed enough of a customer base in early California, that he authorized guns of his design being built in Marysville California. Charles Slotterbeck built doubles (or more) in Lakeport, his business partner Villegas built double rifles in San Francisco, and a gunsmith in Red Bluff has recently been credited with a three barreled rifle recently found in parts at a local yard sale. So double rifles were very popular toward the end of the muzzleloading era, at least on the West coast.
The majority of the doubles I have seen that are rifled are gain twist, and designed to shoot a bullet instead of a patched round ball. The bullet is usually shorter than is common today. So, there are a couple of work arounds. One is to make your gun .45 cal. And find an old replica brass mold from a revolver case set. They are supposed to be .451, but are almost always undersized a little, and shorter than a target rifle bullet. You should be able to size these bullets to fit. Another fix is to buy some steel dies for the proper caliber, and mill the rear of the blocks off and reinstall the sprue cutter.
Hungry Horse