6mm or 8mm.
I know this will probably get people angry if used at AP events, but...
I like the idea of a Weapons Squad. A squad completely devoted to two M240's. A squad leader controlling a pair of M240's (or M60 VN) which have gunners, assistant gunners (feed the ammo into it. AKA A-Gunners) and then possibly an Ammo Bearer (basically a low ranked guy who is along for the ride to carry shit and provide rear security while the gun teams provide support by fire).
Then up the FPS for gun teams. Make the M240's shoot closer to 600fps. Bolt action FPS so that they can actually reach out a bit further, and are a bit scarier to the average player.