I tried this script and got it working, its very nice, thank you for writing it.
I placed the fn_selfactions.sqf part and added the An2_ammo.sqf.
Then I added your part Calamity and found it did add the guns but no ammo.
If you change:
_object addMagazine "100Rnd_762x51_M240";
_object addMagazine "100Rnd_762x51_M240";
_object addMagazine "100Rnd_762x51_M240";
to this:
_object addMagazineCargoGlobal ["100Rnd_762x51_M240",3];
then the aircraft has 3 magazines in the cargo when you spawn it in.
You can then add them to your inventory and use the load ammo option.
Psyfer got a reaction from calamity in Help with adding M240's to Bi-planes