Is there a way to make a gn use at least two different types of ammo, or maybe if you have both in your inventory choose one. The way im thinking of is using the rock-it-launcher ammo type and edit the list.

anyone?

No, each gun uses one type of ammo. You can use a script to swap out the gun for another using a different ammo type (e.g. swap "Assault Rifle w/Regular Bullets" for "Assault Rifle w/AP Bullets").

figured as much, oh well

Or you can use FOSE's SetWeaponAmmo.

Only in very specific situations, where you know there's only going to be one copy of the gun in the gameworld.