[rmc]

"Imperial" in USA is actually not the same as "imperial" in the UK. There are slight differences between all units, but a significant difference with volume. A US gallon and a UK gallon are different by about 25%.

The US "imperial" vs UK "imerial" is why during WW2 the Allies (USA, UK, France etc.) used metric. Cause there's only one cenimeter. (Very important when you're making bullets)./

[friendzis]

I think that most common infantry bullets in allied side were .30in (7.62mm) and .45in (11.43mm) and to some degree 9mm. So yes, to some extent you are right, but note that caliber is inch based