The MAC wasn't really altered, it's a misconception
The reactors, specialy designed rounds, and capacitors are what changed, not the MAC barrel or coils
The reason it was able to shoot 3 shots was because it used recyclers to dump wasted power from the coils back into the capacitors for the next round, it also probably had extra capacitors
And it had a prototype reactor that performed much better than most UNSC reactors, i cba with the details cause i can't remember most of em, but basicly the hotter you ran it, the cooler it got
The rounds were also lighter, not necisarily smaller, they weren't completely solid like normal MAC rounds, they have a depleted uranium/tungsten alloy shell and a core made of shards of tungsten alloy, like a marine's shredder round, it wouldn't go in and come out the other end of the ship, but it'd spread it's damage around non fataly to disable as many systems as possible
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