Why Are Bullet Dimensions Not Round Numbers?

[Felch]

Shape of the bullet matters a great deal, both to match the bore of the barrel, and to minimize air resistance. Round balls haven't been used since the mid 19th century, right around the time that mass produced, interchangeable parts became standard.

[KrazyHorse]

Dry, that's very unlikely, as the calibers of ball guns were not the same as those of modern guns.

Modern calibers were developed when bullets already had today's shape.