>>293742067
You can buy a cheaper gun that uses the same ammo type like a FAMAS for rifle ammo to then use in that modded M4 someone dropped. Otherwise you could also ask a couple of teammates with rifles to drop a full magazine so you can put them on your chest to convert back to boxed ammo that you can then use in the dropped gun. They sacrifice a bit by doing that but if they survived last round then they'll have extra anyway.