Water cooled systems would work best.
Just an example:
The Vickers Machine Gun used in the First world war mainly by the brits, were water cooled. At one time they were fired 10 hours continuously, and no overheat, jamming, or any other problems. That's the miracle of water cooling.

However, they had 5 dudes just to carry one set.
Stand.
Gun.
Other parts of Gun.
Ammo.
More Ammo