Longer naval guns demand more than stretching the barrel

Why could France not simply fit very long 13-inch/60 guns to a battlecruiser design?

The longer barrel would require stronger metallurgy and machinery to control its weight and prevent droop, so substantial development would be necessary.

Making a naval gun longer can increase performance, but it also increases mass and places greater demands on the mounting machinery. The barrel material and construction must keep the tube stiff enough to resist droop under its own weight. Even a small deformation changes the shot's departure angle and undermines accuracy. A French 13-inch/60 proposal therefore required a major metallurgy and engineering programme rather than a simple lengthened version of an existing weapon.