Yes, I used to use mine for an M4A1 (until I changed pretty much every part on the gun - still fits in the bag it's just not an M4A1 any more).
It's long enough for an M249 Para with the stock stowed and also has an extending rifle barrel cover that comes out of the main compartment if you have any longer guns you need to carry (I use it when I'm carrying my M14 EBR for example). I've regularly used one to carry my SCAR-L, an M4, an AKS74U and a pistol all at the same time. Great bags (whichever one you buy).