I'll look into it but have never known that to be a fact. Will post as soon as I can confirm or reject the law.
In no way am I trying to be an ass but you may want to check your game laws. In the US the 8 gauge is outlawed for hunting. That does not mean you can not own one. As for as I know the 10 gauge is the largest bore allowed for actual hunting here in the states. If I could actually use this I would have snapped it up immediately. Great looking shotgun!
I believe that anything larger than 10 gauge was outlawed in the early days of the states establishing their variously named Fish & Game Departments.
The reason being that a lot of winged upland game, but most especially waterfowl had been nearly made extinct by market hunters in the 19th & early 20th Centuries as a result of the insatiable demand for wild game in the restaurants of EVERY big city in the United States.
Teams of hunters, mainly family members, would creep up on the shorelines of any waterway where migratory waterfowl rested at night armed with the biggest bore shotguns they could handle. This included 8 bore double & single barrel guns all the way up to 2 bore double & single barrel guns.
When those big bore guns proved to be insufficient to the task of killing large numbers of waterfowl at one time, plus the recoil was punishing as hell on men's shoulders, most market hunters switched to punt guns mounted in heavily reinforced skiffs. These punt guns were nothing more than crude, medium bore (2") cannons with amazingly long barrels that extended from the reinforced stern of the skiff all the way out past the bow, usually by several feet. They were aimed at the middle of a dense flock of resting birds on the water by moving the skiff to point the muzzle of the punt gun. Incredible amounts of powder, and shot were used per shot, with over a hundred birds being killed/maimed on a single shot.
The Chesapeake Bay has a very rich history of punt gunning for market hunters. With anecdotal stories of famous family owned guns that were so powerful, and loud that even after they had been outlawed, the families hid the guns from the authorities so that they could continue using them. Much like the culture of moonshining.
This is why nothing larger than a 10 bore is currently allowed in the United States for shooting anything with a centerfire shotgun. A lot of muzzleloading regulations are very vague when it comes to spelling out what is, or is not allowed in a particular state. Some states specify quite clearly that the same rules that apply to centerfire hunters, apply equally to muzzleloading hunters. Other states grant much more leniency towards historical correctness with muzzleloading, allowing things that are not allowed during the centerfire seasons.
An 8 gauge smoothbore MIGHT be legal in a few states, but I don't think I would bet my freedom on using one to hunt with unless I had something in writing with a real, notarized signature from a high muckety-muck in the Department of Fish & Game from the state in question.