Robgunbuilder, Ed Hubel, and a gang of more-than-slightly demented gunheads over on the Accurate Reloading forum are building .720 caliber rifles. The ATF is okay with that, since they're legally shotguns. In the beginning they were using modified BMG brass, blown out straight, and with a rim added. They got RMC to make a run of custom "FH" brass, and Ed's been playing with plastic cases too.
12 Ga From Hell -WOW - Topic Powered by Social Strata
According to the ATF, anything over .50 is verboten. Unless it's a pre-existing cartridge like the .50-110 (.512 diameter) or .700 Nitro, which are grandfathered in. And they've ruled that rifled barrels on shotguns are okay, and still shotguns. And that even if you rechamber a shotgun to shoot a metallic cartridge, it's still a shotgun as long as it still shoots shotgun shells. None of which is particularly consistent with the letter of GCA '68, but we're talking bureaucrats here, not gunheads.
So, the 12gaFH is just like a .50 BMG. Except it's BIGGER! BWA-HA-HAAA! <evil laughter ensues>