That type of gun got about. The brech loading type were used on HMS Warrior and black Prince, and as said breech and RML (Rifled Muzzle Loader) for costal mounts. Two brech loaders are on the Cenotaph in St Helier Jersey, so guess how I recognised the type? :-D The Armstrong of this type were constructed of a number of tubes, the main being steel then wrought iron shrunk on. Armstrong made breech loaders, but reverted to muzzle loaders as the Britsh Goverment of the time decided muzzle loaders were cheaper. Sound familiar? So the muzzle loader you have in Norway may be later than most of the breech loaders.