Noting that the Gatling Gun arm he receives that's standard as soon as he joins your party looks semi-similar to a M134 Minigun, I'd have to say it'd be FFVII's world equivalent of 7.62x51 NATO. Preferably Full Metal Jackets or Armor piercing rounds for Shinra troops and robots and monsters with really dense hides. A big thing about Barret's gun arm I've been wanting to ask. How does he even eat food or write or do anything with that arm? I mean Advent Children is there because he also has a prosthetic right arm but he never mentioned it or showed in FFVII.