The M249 SAW is a light machine gun firing 5.56x45mm NATO rounds. It can either be belt-fed with 200Rnd. M249 Belt, or use traditional magazines 30Rnd. STANAG and 30Rnd. STANAG SD to take advantage of more commonly found ammo.
The M249's main advantage is that it shares the same stats as other 5.56 assault rifles, but carries a lot of ammo in a more compact package. Each box of 200Rnd. M249 Belt holds 200 rounds in two slots, compared to the 60 rounds that two 30Rnd. STANAG hold. It's immensely useful for killing zombies due to its large amount of ammo and relatively quiet shot volume.