The diamondback cannon sure looks like a modern design to me.
First let me say that I certainly do not want to dispute anything from your informative posts on this subject and want to be safe AND legal. I was just curious how the ATF might view this apparently modern black powder cannon design.
So I just spoke with the guy that builds and sells these cannons, a nice fellow named Mark. Mark says he spent a year going back and forth with the ATF trying to determine their legality. The removal breach was a major sticking point. Although Mark left that design feature in place, he modified his accompanying literature to state that breach removal is strictly for bore cleaning and observation and NOT for powder loading. He also designed a muzzle loading tool, basically a long rod with a powder cannister on the end. You load a measured amount of loose powder in the cannister, close the cannister, insert it in the barrel via the muzzle, run it all the way to the breach face, and turn the rod counterclockwise. This releases the powder and places it where it is supposed to go. The loading tool accompanies every cannon he sells.
Apparently these actions allowed him to convince the ATF to treat this gun as a muzzle-loading black powder gun.
Mark says he has shipped the cannons all over the US. He cautions to follow all local ordinances but is steadfast in saying hat the AFT considers his cannons in the same category as modern black powder muzzle loading firearms.
Does this sound on the level to you? I am by no means an expert.