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. :
M249 light machine gun - Wikipedia
S&T M249 SAW E2 AEG BK – S&T Armament (Smart Team)
Pimp my gun : M246 by LenRilly95 on DeviantArt
M247 machine gun - Weapon - Halopedia, the Halo wiki