Would the British 15-inch/50-calibre gun have been ready before the early 1920s?
Short answerThe hardware might have been ready around 1919, but fitting and working up the complete shipboard system would probably extend into the 1920s.
NotesThe 15-inch/50 project had been under consideration long before postwar capital-ship studies made it visible. Manufacturing and mounting a gun would still precede proving a reliable turret and learning to use the weapon aboard ship. A 1919 hardware date is therefore plausible, while operational maturity would come later. This distinction keeps alternate designs from treating every advanced drawing-board weapon as immediately deployable.