The 18-inch Mark I may have been a shortened longer gun

Was the BL 18-inch Mark I only a test bed for the feasibility of an 18-inch naval gun?

It may have begun as a 42- or 45-calibre design and been shortened to 40 calibres so it could fit HMS Furious, rather than being conceived only as a test weapon.

A 40-calibre length is unusual beside the Royal Navy's more familiar 42- and 45-calibre design practice. Descriptions suggesting that the muzzle end appeared cut and polished support the suspicion that the weapon was designed longer and lost several feet to make installation aboard Furious possible. The hypothesis also links the completed gun to larger 18-inch weapons considered for later capital-ship projects.