CCMonster31, The movie being being what it was, useing a fictional character that didn't even have anything to do with the actual happening, would not have cared if the double rifle had shot nothing but smoke, which is probably the case with a movie prop gun. Besides, most likely Michael Douglas wouldn't know a muzzle loader from a machinegun, in fact, the hauwdah pistol he used in that movie was a caplock muzzle loader, and he was the dirrector, and producer, of that movie!

In fact many professional hunters used black powder rifles to hunt way up through the first 10 yrs of the 20th century, or so! Especially the elephant hunters who used the large 2, and 4 bore muzzle loaders, and 4, 8, and 10 bore NFB ball double guns, and explora doubles. The .450 bores came out way back in the mid 1800s, in black powder double cartridge rifles, then most were later re-proofed to NFB, the 450BP 3 1/4", was the exact same cartridge as the NFB, and Cordite version, and some simply didn't bother re-proofing, but simply used the new ammo in the old rifles. In 1898 the rifles smaller than 577 were considered small arms, but the .450s were regularly used on lion. The guns weren't considered to be big bores untill 3 bore and larger.

None of these facts have anything to do with this movie, it was strictly Hollywood!