Metric navies still clustered around established gun calibers

Why did metric navies build guns close to British and American inch calibers instead of using round metric sizes?

Because established gun families and borrowed technical standards mattered more than choosing tidy metric numbers.

Naval gun calibers clustered around familiar 12-, 14-, and 16-inch families. Navies did not choose the numbers in isolation from existing weapons, industrial practice, and foreign designs. France drew gun technology from several sources and consequently produced less regular caliber patterns. Metric measurement did not erase inherited technical standards.