Yes, I have shot an 8 bore shotgun. Bit of an anticlimax. Oh, it is definitely not a .410, but kicked less than a modern 10 g goose gun. BIG, heavy gun, and 8s were black powder shells.
Yes, there were 6 bore shotguns, but they were muzzle loaders. There WAS a 4 bore catridge shotgun- I have an Eley 4 g shell in my collection. WHY they made a 4 g is the real question......
PS- 10 g is the legal max for hunting in the US.
What we have here is... failure- to communicate.