Choosing railgun calibre around mass and loading

Why might a railgun use an awkward calibre such as 254 mm instead of a round metric number?

Calibre can follow requirements for projectile mass, length and loading. The discussion also argues that sufficiently heavy 10-inch ammunition could make a 12-inch system preferable.

A suggested 10-inch railgun prompts the argument that a shell of that size would impose loading demands close enough to a 12-inch weapon to favor the larger calibre. A later question challenges apparently awkward metric dimensions. The response argues that workable shell size and rate of fire can matter more than a round metric number. Another panelist adds that a specified projectile mass and maximum length leave diameter as the remaining adjustable dimension. The choice of calibre therefore depends on the projectile and handling requirements assumed in the design.