The Marine Corps disagrees with your statement about turning things over after 300 meters. The Army agrees with that statement, which is why they instituted the SDM program so that each Squad would have a rifleman to engage targets out to 500 meters or 600 yards (depending on the range they train and qualify on). That gives a 9 man squad 3 individuals who can engage out to 500 meters, the SDM and the two SAW gunners.
SOCOM usually runs in much smaller teams, so individual rifles count a lot more, hence the development of the 6.8 SPC and the 0 to 500 meter lethality envelope.
Machine guns are awesome until you have to carry one.