Accurate gunners NEVER fire on fully auto. When I was the commander's gunner behind a .50 cal cruising in a humvee around Baghdad and (more often) Mosul, Iraq... You fire controlled 8-10 round bursts or 3-5 round bursts if you aren't trying to go nuts.

Perfect aim and fully auto are mutually exclusive. That is why the M16A1, which was used in Vietnam as the standard issue rifle, was replaced with the no longer fully auto M16A2. The A2 has safe, fire, and 3 round burst. The fully auto selector was removed as soldiers just wasted a lot of ammo and burned up barrels.