Interesting thread here, just a thought came to mind for those of us in the USA:
I believe (not positive) that any cartridge handgun cannot exceed .500” so any .attempt at 577, etc. requires a special application, review, and permission from the BATF. (OMG, a destructive device don’t you know)
Of course this does not apply to those existing antique Howdah pistols and modern muzzle loaders.

- Mike