Why did Royal Navy gun calibres rise in roughly 1.5-inch steps?
Short answerThe apparent pattern may reflect practical shell-storage and handling arrangements as well as the trade-off between a longer barrel and a larger shell.
NotesRoyal Navy calibre jumps may reflect the steel frameworks used to move and store shells aboard ships and at overseas bases. This logistical explanation is tentative because it had not been independently verified, although other evidence appeared consistent with it. A 50-calibre 12-inch gun requires about as much barrel length as a 45-calibre 13.5-inch weapon, which can throw a heavier shell. Larger calibres were already under development when HMS Dreadnought was built, but available 12-inch guns allowed construction to proceed without delay. The move to 13.5 inches balanced manufacturing readiness, barrel length, shell handling, and ballistic effect.