"For training exercises, the M249 is used with the M15A2 BFA (Blank Firing Attachment), which is the same one used on the M16A2. For the M249 to function when firing blank rounds, the BFA must be used."
http://en.wikipedia.org/wiki/M249

Did you notice this attachment?

"Blank Firing Adapter (BFA)

For years Marines have been complaining about the BFA on the SAW. It never seems to fit right. This means you don't have a solid block on the end of the barrel to cause enough blowback to keep the weapon moving through the cycle of operation. What I ended up doing was pulling the cotter pin out of the BFA to allow the block to sink deeper into the BFA, thereby getting a tighter seal on the end of the weapon. Now this is not a problem as long as you have a multi-tool, or pair of pliers available during training. Once the BFA latches on securely, it does not want to come off."
http://usmcweapons.com/articles/M249/M249NF.html