Hi Lew,
According to Howard Blackmore, there are at least 3 guns in existence. An iron gun and a brass gun are in the Tower of London, and a brass gun is in the Tojhusmusset in Copenhagen. The gun was not a success but 2 were carried on a British naval expedition in 1727. Nothing was mentioned about their use in the expedition's log. The brass gun in the Tower is a flintlock but the other 2 known to exist are ignited by match (like cannons of the time). Their bores range from 1.2-1.6 inches.
dave