Mag-fed SAW's work better than belt-fed in a fast moving combat environment. Belts hang-up, get twisted and are hard to transport even neat plastic boxes, especially when you have 200rds of ammo, does weigh a bit too. Mag-feed on the M249 has always sucked and the position makes it hang up a lot. The USMC wants an easily portable unit that integrates better in a squad with M-4's and makes shoot and scoot easier.