This is the closest I can legally get to an M249 SAW. It's a civilian transferable lower with the MCR belt fed upper and the same trijicon scope that the SAW uses. :
Raptor Two Point REFLEX Sling MK2 - Raptor Tactical
Blue Force Gear Vickers M249 Saw Sling | w/ Free Shipping
A&K M249 PARA SAW - Support Gun on a Budget | Airsoftology Review - YouTube
FN M249 Front Sling Mount Kit: MGW
U.S. G.I. M249 WEAPONS SLING, 2 PACK | eBay
USGI M249/M240B Machine Gun sling
Vickers M249 SAW Sling Coyote Brown - BLUE FORCE GEAR
Vickers M249 SAW Sling
USMC M249/M240B Heavy Weapons Sling – KommandoStore
Blue Force Gear Vickers M249 SAW Sling Coyote Brown Features a larger pad in the rear and Nomex front webbing to prevent burn through VCAS-SAW-CB | Cheaper Than Dirt
U.S. Military Surplus M249 LMG Weapon Sling - 708494, Gun Slings at Sportsman's Guide
Guarder M60/M249 Sling 2013 Version | Popular Airsoft: Welcome To The Airsoft World