My father served first in the infantry during WWII although he didn't talk about it much. Once his service was over, he re-enlisted in the Merchant Marines, where he was an anti-aircraft gunner.
I've still got one complete round, minus the gunpowder, that he brought home with him. I don't know what size it is but it's stamped with the numbers 42 and 659 on the bottom of the casing. The shell (slug) itself is about 3 1/4" long, brass tipped steel and is very heavy for it's size.
Also, one of my brothers was an Army Medic in Korea.