Why French naval gun calibres look unusual

Why did the French Navy use such unusual-looking gun calibres?

Many figures came from translating inch-based gun sizes into millimetres and then applying inconsistent rounding, which left apparently awkward values rather than a neat universal series.

The unusual calibres grew from changing measurement systems rather than random design. French practice converted inch gun sizes into millimetres and sometimes rounded the result upward. That process helps explain treaty-era references around 5.1-inch and 6.1-inch limits as well as weapons labeled 138 mm. Other values remained closer to inherited figures instead of becoming clean tens. The watch point helps readers compare national gun labels without assuming every unusual number represents a wholly separate tactical concept.