Not meaning to be contrary here, but there was a cased pair of London Guns retailed 12-bore rifles on Greener GP actions that changed hands a couple of times in Australia maybe 10 to 15 years ago. Google Images is unhelpful but I remember ivory grip-caps and blue baize case-liner. Pretty sure they were featured in a thread on these forums somewhere.

There are four such rifles in regular use for the Bore Gun match at our local Big Game Rifle Club. Two were built just last year and are both on the take-down action. Those two are chambered for brass case, not sure about the other two but one is definitely rifled for round ball. The latest two shoot 680 to 750gr Fosbery projectiles at ~1100 fps all day long.

Its pretty common practice amongst BGRers Australia wide to fit fully rifled 12-bore barrels to Greener GP actions.
Yes, they can be a ponderous pig if the barrel profiler is lazy, but with a heavy breech section behind the forward hand and plenty of taper to a relatively light muzzle, they sacrifice nothing and handle just fine.

Nevertheless, what DD says is true: Graham Greener definitely says do not use any of these shotguns for centerfire cartridges.
But a lot of people have ignored that advice... !

There is at least one .577 Light Nitro Martini in Australia's Top End that started life in Nhulunbuy, and has accounted for plenty of buffalo.
A second one was in the mix before covid IIRC, but don't know whether it was the GP or a Martini Henry action.