When I first learned of Ed Hubel's "12 Gauge From Hell" the Voices said, "you could shorten one of those and chamber a barrel for a Saiga-12." For those who aren't familiar with the 12gaFH, it's a .729 caliber wildcat cartridge. The forming instructions begin with, "first you take some .50 BMG brass and blow it out straight..." You'd have to shorten one considerably to get it into a Saiga-12, though.
However, it just came to my attention there's another big bore that might be might be more appropriate - the Winchester .70-150.
"The .70-150 Winchester is a hypothetical American centerfire rifle cartridge.
Appearing on cartridge boards of the Winchester Repeating Arms Company in 1888, it may have been nothing more than a display novelty based on a brass 12-gauge shotshell (rather than the paper cases common at the time, or plastic frequently seen today), shortened and necked-down slightly. Only one rifle was ever chambered for it, and it was never offered commercially."
Bullet diameter .705, neck .725, rim .870, case 2.18, OAL 2.63, recommended twist 1:16. 900 grain bullet.
The levergun and cowboy action forum hits say Winchester made more than just one rifle, and that the Model 1887 shotgun (the one Arnie carried in Terminator 2) could be had in .70-150 through 1905.
Either way, Winchester *did* make a .70-150, and it can be formed from a 12 gauge brass case, and it should fall into the same "destructive device" antique exemption as the .577 Snider and the big Nitro Express cartridges.
So, if your .500 S&W or .50 Beowulf is starting to seem a little weak in the knees, and you haven't fused your shoulder socket yet, there is a bigger, badder step you can take...