A friend had to classify some zimmer shutezen held by the Imperial War museum. Granted they were antiques but had to be tested and classified for some reason.
They have an 8" barrel and fired flobert caps, all produced under 6ft/lb so were designated airguns. An explosion creates a compressed gas in a confined space. The release of gas create MV within the range of an airgun.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane