Sounds like it is close to being a 4 bore. Might be a light punt gun but more likely what used to be known as a Marsh Rail or Bank Gun. They were used, often in the East Anglian fens of England, and poked over the banks of the drainage ditches and fens to shoot groups of water fowl on the water. Not used for standing shots from the shoulder but rested on the banks. Used by Market Hunters rather than sportsmen. Still legal over here in England. Lagopus.....