Caliber of Barret's Gun Arm?

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.

They never tell us much about the actual hardware of the gun; it seems that all his gun-arms use the same generic type of ammunition since he never has any issues after changing to a new weapon.

In combat, he always fires in bursts, but each burst is only about as damaging as a single round from one of Vincent's handguns or rifles. To me, that suggests Barret's guns aren't actually all that powerful, though I may just be reading too much into a gameplay mechanic.

As for Barret's day-to-day life prior to getting his 'Advent Children' transforming prosthetic hand... in the game, he tells us that he originally used a prosthesis after his hand was destroyed, but eventually discarded it and had the gun mount grafted in. I'd assume he got quite proficient at using his left hand for daily essentials, but he never became that good at writing - after all, when Cloud spots some Avalanche graffiti in sector 7, warning about the impending end of the world, he comments on the terrible handwriting that looks like it was written by a child. I aways assumed the writing was Barret's, since he's the Avalanche leader and has lost his dominant hand.

I always just assumed that the lower damage of barret's bursts were due to pretty poor accuracy. There are probably at least two, perhaps 3 types of ammo for the various weapons, judging by the various hit effects you can see.