I think that the reason for that may be political pressure due to the fact that a felon could have a BP shipped to his door and buy a cylinder to convert it to an undocumented unmentionable with no paper trail. However, he’d have a pretty hard time finding ammo, or getting reloading supplies in today’s market. Plus it would hardly be the weapon of choice, but I think that is one loophole that they are trying to close.
Check the American SHOTGUN pistol sponsor's wares and think about it.