In rural Newfoundland, that would be called a "Seven-eighths Gun" and would have been used birding until relatively-recent years.
Something that size would also have been useful on Potheads (Pilot Whales, not Marijuana-smokers!) should a pod run in close to land.
The Lock appears a conversion from a Short Land Musket (Second model Brown Bess).
The Wedding Band at the Breech is a bit unusual; most of these had an Octagonal barrel which changed to 16 sides, THEN the Wedding Band, THEN a tapered round barrel to the Muzzle, total length usually 46 inches. THIS one is OLDER, likely an 1820s conversion from parts surplus after the Napoleonic Wars into a cheap gun for Colonial trade. In Newfoundland this would have been a 6-dollar gun, but the dollars would have been Spanish Milled Dollars: pieces-of-eight.
The Hammer appears to have been changed-out on this gun. As made, they generally were surprisingly well-fitted. Time, oxygen and salt-water do take their toll.
BTW, the HOLE through the stock is for a "Punt-line". A heavy line was run through the hole and seized back upon itself, spliced strongly and the other end fixed securely to something solid in the boat. If the boat capsized or, more likely, if you dropped the gun from sheer recoil, you could recover that expensive gun.
Guns such as this commonly were shot with an OUNCE of powder and a quarter-pound to a half-pound of shot! In local language a "shot of ducks" is about 30 birds: the number which you should be able to take with a single shot from this monster.
PLEASE don't ask me to hang onto one while it is fired!
Most of these were very cheap London guns, but a lot also were made in Birmingham. Two hundred years of salt-water will play Hob with proof-marks.
A MARVELLOUS old chunk of pure Canadian History!
Hope this helps.