I didn't know they were still legal in this country.

Google punt guns and there is a lot of information about them.

I found this snippet:-

Some big guns gained almost cult status.’Irish Tom’ was made as a muzzle loading punt gun. It was bought from an Irish market gunner in the early 1930s by WAGBI founder Stanley Duncan. He had it converted to breech-loader by Greener and started using it in 1936. When Duncan had finished with the gun he sold it to the actor James Robertson Justice. It was rediscovered and restored in the 1980s and given to BASC to display at their HQ. Irish Tom has a barrel made from Whitworth steel, weighs 300 lbs and fires 50 oz of shot propelled by 10oz of black powder. The gun is 14 feet in length.