Why Dunkerque could not simply receive 15-inch guns

Did France consider arming Dunkerque with 15-inch guns, and could they have been fitted?

The idea was considered, but France first needed a completed gun and production system, neither of which was available for the ship.

Changing calibre was not simply a matter of altering a drawing or enlarging a barbette. France would first have to complete a suitable gun, turret, tooling, and associated manufacturing arrangements. That equipment was not available when Dunkerque was being built. Industrial readiness and schedule therefore shaped the armament decision as much as the desired specification.