I have seen Punt Guns in museums in The UK and seemingly in T2K: R/- (in The UK). However 'just' a pipe and...
"In the United Kingdom, a 1995 survey showed fewer than 50 active punt guns still in use. The Wildlife and Countryside Act 1981 limits punt guns in England and Wales, and in Scotland, to a bore diameter of 1.75 inches (44 mm) (1 1/8-pounder). Since Queen Victoria's Diamond Jubilee in 1897 there has been a punt gun salute every Coronation and Jubilee over Cowbit Wash in Cowbit, Lincolnshire, England. During the Diamond Jubilee of Elizabeth II, 21 punt gun rounds were fired separately, followed by the guns all being fired simultaneously".
From
https://en.wikipedia.org/wiki/Punt_gun
It is 'remembered' from an old White Dwarf magazine Call of Cuthulu scenario but they said that Boyes anti-tank rifles could be bought / owned on a shotgun licence as they are smooth bore and thus 'OK' under English & Welsh Law. (The mag would date back to the 80's?). Buyer Beware!