I have both the original chambering, and one that takes .410 3" regular ammo. The original one requires "blown out" .303 British cases. Potashminer sent me a bunch of brand new PPU boxer primed .303 cases. They all blew out perfectly, no splits. Other brands may or may not work so good. I tried others and some worked, some didn't. I think I'll stick with PPU. I bought some PPU and found them to have Berdan primers, so I drilled the pocket out with a "C" letter drill and used shotgun primers.
The 3" chambered one, I can use regular plastic (or paper, I suppose) hulls or 444 Marlin brass. I have posted about my experiments with round balls and .410 actual cast bullets poured with a LEE .410 mold. It's been a journey of experimenting, failing, succeeding, and learning always.
At the muzzles, both measure .410, and are likely .410 the whole way back to the chamber. They are fun to shoot, a decent bang, next to no recoil with my loads, and, well...reasonably accurate.