i can only see using the sling on a M249 as stabilization if it's hip-fired... and yeah, the mkII barrel is kinda crappy, but in its defense, not entirely. the main problem i see with it is that the barrels get burned out faster since they removed the gas collar and stuck that solid piece of drek on there.
at short range with any weapon on burst, yeah, the likelihood of a first-round hit is very high. i dont see it doing much extra damage though if that's the only round that hits. granted, it does still reduce dodge due to hot lead on either side of the poor bastard, but if the gun jumps too much to provide follow-up hits, why fire sustained at all, unless to provide suppressive fire?
two very important truisms i live by:
1) the deadliest thing on the battlefield is a well-aimed shot
and 2) consistency = accuracy