Possession of a 2, 4, 6, 8 bore gun is not illegal...the shooting of the 2, 4, 6, 8 bore gun is not illegal...the hunting of birds with a 2, 4, 6, 8 bore gun is illegal, with some state/provincial exceptions so for argument's sake lets just say it is...
I've shot both 4 and 8 bore BP rifles and shotguns and a 8 and 10 bore duplex (smokeless) rifles and shotguns...Loud, massive muzzle flash, but only a very slow, heavy push instead of a sharp recoil...Wouldn't want to shoot 10, 15 rounds in a row with them...A friend has a 10 bore British made Cape Gun - one barrel smoothbore the other fully rifled...
The primary shotguns of the Wells Fargo Stagecoach and Express Line, was usually a W.W. Greener, 10 bore, 30" or 32" shotgun and the short barrel models (and sometimes 12 bore) were carried by sheriffs or town marshals etc. as they were easier to handle inside a building....The drivers wanted something with reach, and the longer barrels allowed more powder to burn inside the barrel which of course creates higher velocities and usually longer range...
The close up, non firing shots were done with the true 8 bore gun, IIRC from a couple of articles, some of the closeup firing shots were done with the original but all the rest just tricked up 10 bore doubles...I'll find the article and post it!