Fixed position, with probably no more than a 90-degree field of fire, belt-fed. Standard loading for .30 was 110-rounds, as I vaguely recall, and .50s were 105-round belts in each can. Shoot longer between reloads than magazine-fed guns.
Mobile, magazine fed is the only way to go. You play hell tripping over belts and all that, aside from the inherent near-impossibility of shooting full-auto on the run. Move, stop, shoot; move, stop, shoot. BAR *good*!!!
As usual, it's situational and tactical...
Contrary to Hollywood crap, the best way to fire machine guns is to say to yourself, "Fire a burst of six"; pause; "Fire a burst of six"...The barrels last a lot longer. It's real embarrassing to shoot at folks 200-400 yards away, and the rounds from a shot-out barrel hit the ground about 50 yards out. That's when your own life-expectancy approaches zero and your insurance agent cringes.
Obviously, if you're being faced with a large and hostile group at close range, barrrel life becomes a lesser priority.
FWIW, Art