The most popular black powder muzzle loaders used for hunting large game animals in the U.S. today are .50 caliber inline rifles. They're also available in .45 and .54 caliber and many states do have special muzzle loader firearms hunting seasons dedicated to using any modern or traditional black powder muzzle loader.
Every state is different and has their own regulations, but .44 and 45 caliber saboted bullets are the most popular. But many hunters do hunt with bore size conical bullets and patched round balls too, and also with flintlocks.
Modern inline percussion rifles are preferred because they have a fast twist barrel for superior long range accuracy shooting the saboted bullets, and because they're designed for mounting a scope and for ease of cleaning since they have a removable breech plug. Inlines are also usually rated to be loaded with up to 150 grains of pelletized powder which provides some additional range and lethal energy for hunting large North American game animals.
