[smokingfrog]

It can be fed from both linked ammunition and STANAG magazines, like those used in the M16 and M4. This allows the SAW gunner to use a rifleman’s magazines as an emergency source of ammunition in the event that he runs out of linked rounds.

https://en.m.wikipedia.org/wiki/M249_light_machine_gun

[Future Snake Eater]

That's an M249 Squad Automatic Weapon (SAW), NOT an M240B. SAWs fire 5.56mm ammo, M240Bs fire 7.62mm ammo and they are belt-fed only.

SAWs can use magazines, but that firing method isn't terribly reliable.