Hmm...
player ammo "m16"
works fine.
Are you sure you don't have an ammo box or anything else named "ammo"? If yes, change the name to something else or even better would be to not use names at all if possible.
That could make things go wrong.
BUT:
These work for the BIS Camel:
BISCamelMGun
BISGrenadeCamel
so it would be:
_ammo = plane ammo "BISCamelMGun"
There's an example attached, unzip it and copy the folder to
Operation Flashpoint/Users/Missions
And load
Desert Island/ammotest
in the editor. Use the radio...