I've been playing more and more support lately. I agree that sighted 5-10 round bursts depending on range from prone is the best deployment. But the best usage is just as the name says, in support of a squad. I think more so than maybe any other class it depends on the rest of the squad to be effective.

You really aren't toe to toe, one on one superior in firepower to riflemen unfortunately(TOTAL reversal of real life IMHO, modern infantry squads are organized around their SAW gunner). So think of how to best use your gun to empower your squad mates. Suppression obviously. The first seconds of contact have me laying down half a magazine on whatever cover the enemy appeared from while my squad mates maneuvers to their chosen position for the engagement.

If it's a flank-able enemy position, I empty a magazine then switch to nades to deny the enemy a frontal advance. Also if it's a corner, I lay down fire as my boys move along the joining wall until they are close enough to nade the corner and roll around it. Have to be careful though because the LMGs are deceptively inaccurate and friendly fire happens sooner than you'd expect as teammates advance into your suppression. Too bad the game doesn't model true MG trajectory fire and you could shoot over the heads and drop the fire in front of them.

At range, I try to pin enemies prone(their fire instinct under MG fire) so snipers and riflemen can pick them off. Obviously this makes me vulnerable to return rifle sniping so this is a instance when I slither around as I don't really need to be accurate to get their heads down.

More often than not though I try to be the tip of the spear, suppressing the enemy's ability to move in the open(sprinting infantry targets is the only kind the LMGs seem superior to the rifles in engaging). In very short order though I will be the focus off all enemy return fire if I'm suppressing correctly and I die. But hopefully a medic is nearby and by successfully tying up a few enemies for awhile, my squad mates will be able to capitalize on my death.

Such a diversion is really great with a mobile SL. The entire enemy squad focuses on the support fire. They flank around taking themselves out of the action temporarily. Or they move to adjacent defilades in my field of fire to nade or charge me point blank. They basically shift their axis of attack towards me and expend considerable time and energy(often to the neglect of their true objective). Meanwhile a mobile SL will be arcing way behind me and around their advance on my position.

By the time they get me and I respawn on the SL, he's on their flank or in their rear beyond their engagement range. At that point I can either repeat the process with devastating flank fire, or serve as a temporary rear guard and lay in ambush in case the squad that charged me realizes they've over-advanced and backtracks.

In any case don't expect to be one-on-one effective. Know your suppressive fire and your deaths are helping the squad achieve their snipes and maneuvers.