To fix the issue of ammo not loading into guns is due to a mismatch between the gun and ammo calibers. To fix this, open the ArmAGeddon mission file (ArmAGeddon.Altis.pbo) in Notepad++, then search for "#include "All_Array_pos_loot.sqf"", once you've found this, look just below for "Possible_magazines" and "Possible_weapons".

Then change
"10Rnd_762x51_Mag" and "150Rnd_762x51_Box_Tracer"
to
"10Rnd_762x54_Mag" and "150Rnd_762x54_Box_Tracer"
(There should be two of each in total, one of each in the "weapons" and "magazines" categories)

And all you're all done! You should be able to open the map and load the guns!