Umm... I would have to take a guess and say. 25mm? I have similar casing and links from a Ship gun I fired off the USS Essex back when I was stationed in Spain.
Next time i get over there i will look for the markings. My neighbor served as a United States Marine and is no longer with us. His Son's do not know what they are.
Not an authority by any means,used in a couple applications.My son loads 'em (MALS 13).This is him leaving for the stan 4/17/12 ,he forgot a couple at home.