The M4's are actually pretty common at the airfield. I headed north to the NW Airfield to raid the military tents. There's a fuckton (And I mean a fuckton) of ammo there. I also got my silencer and a few other things there.
Do you know about making rags to use as bandages? Just take off your shirt if you're that desperate and turn them into rags, could save you. Good luck man!