I haven't seen this exact situation, but have seen a lot of rebored/rechambered guns. It was likely a 14 bore gun that was rechambered and rebored. I don't know that the statement about a 14 bore with heavy barrels being intended for heavy shooting rings true of an English shotgun. A 14 bore was not up to a 12 in power and had a smaller bore. Not a gun in that time that would be built or chosen for heavy shooting of waterfowl etc. That would have been a 10 or 8. Typically reboreing and English shotgun is generally not practical as they tend to have very light barrels compared to other manufacturers. Also, the bores as rebored are pretty large for a 12 bore, especially of that period. I will throw out a guess. It was originally a 14 bore rifle and was rebored to the smallest ID that cleaned up the rifle barrel. That would explain the heavy barrels and the overbored size. It was a common practice over the years to take rifles with ruined barrels and convert them to shotguns. Very common with rook rifles to .410 or 28 ga, and I have seem quite a few large bore doubles bored to 20 ga etc.
