A few custom gunmakers have produced 2 bore shoulder guns recently, although it would be hard to see a logical reason until somebody invents a bigger elephant. Or if the owner has a preoccupation with the size of something completely different. Punt guns were larger still, up to around 2in. bore, but they weren't shoulder fired. They were supported either by a spring trunnion device or breeching ropes around the stem of the punt. The nearest some came to the shoulder was a sort of hinged wooden spade under the shooter's chest.
Back in the muzzle-loading days, when one shot was all you were liable to get, the 2 bore for African game had some point. But Sir Samuel Baker, the only person I heard of using one for practical reasons, heartily regretted it and said he never shot as well with anything after using them.
A four bore, by contrast, was fairly manageable if it had the weight of a heavy double. When something monstrous is coming at you, recoil ceases to preoccupy you. They were largely supplanted, though, by double 8 bores, both rifled and smoothbore. Some of the early ones were also very heavy, but they came down enough in weight, with moderate loads, to be quite comfortable to shoot, and even the smoothbores gave groups that would be considered good with modern shotgun slugs.
So number one in my choice would be an 8 bore hammer rifle, chambered for the brass cases. The greatest strength is produced with back action locks and the rotary underlever locking system.
Second is one so rare I have never even seen detailed dimensions. There was a .30/.310 round, and the measurement of some Cadet cases I have suggests that it was simply a .308 inside lubed or jacketed bullet loaded into that case, for an appropriate bored barrel. It is also on the British government's list of freely ownable antiques, if it was made up to 1939, which I am sure they all were... if there actually ever were any. I enjoy telling the police "No, I don't need a licence to acquire it. I already have it. I just want a licence, on account of being law-abiding, to shoot it. N0, I don't need a licence to buy cartridges..." They are pretty used to that from me, but it sharpens them up and improves the breed.
Third is nothing very out of the ordinary. Just a .303 sporting rifle built on the early Long Lee-Enfield rifle which never had a charger guide, to the standard early twentieth century British gunmakers made them. Or a barrel blank and derelict action to make my own.