I would suppose that at range, one pull of the trigger on auto
mode, would mean there goes your rounds, whether you have 3, 5, or
10. But I have used the auto mode at section l/f. Gimme single shot
anytime. 
Another thing to note, auto mode does wear out the bolt faster, so
technical wise, not many will like it. Those who've handled the SAW
and GPMG, they are automatic weapons by themselves. Hold your GPMG
tight when firing, the recoil can bring your shots everywhere, i
mean everywhere. I nearly lost control of it once when I fired it
while it was secured to a pintle on a BX.