For those who are familiar with Battlefield 3, Heavy clip is like a gun attachment. You nerf that and it throws off weapons behaviour for all the other guns in the game.
So if you want to nerf the gattling, then nerf the gattling only. There's seem to be lots of good ideas thrown around by everyone else, why not use a bit of all of them? Damage per round, rate of fire, magazine size, arc, max range. All of them can be tweaked to lessen its effectiveness.