I've made up the following rules to cover this;
HMG and LMG ammunition is counted in ‘BURSTS’. Each player will start with a number of ‘Bursts’ for his platoon. As his HMG/LMGs fire then 1 burst is crossed off his Burst tracker.
HMG/LMG Burst; firing 3 or more shoots at one time or 1 shot at a building.
LMG Double Tap; when an LMG firing 2 shoots at one time. A double tap doesn’t count as a burst BUT does count as automatic fire so the target must take a nerve test.