I'm not sure if it's possible, but it would be nice to be able to change the type of magazine loaded into a weapon in the Arsenal. Currently, the only way to do this is to add the desired magazine to the unit's inventory, "try" the loadout, and then change magazines via the action menu. Since there is no way to "try" a loadout in the Eden editor, there is no way to change the type of magazine loaded in the editor.
This would be nice for weapons that offer different types of magazines. For example, the MXSW is loaded by default with a 100rnd Tracer magazine, instead of the standard 100rnd mag. If you don't want your Autoriflemen to be shooting streams of tracers from their first mag, you have to create the loadout in the arsenal, change the mag via "try," and then load it in the editor. Rifles that use 5.56 are another weapon that would benefit, since there are several colors of tracer available for both the Tracer magazines and the standard mags. Finally, mods that have many options for ammunition (RHS, HLC, and others) would benefit greatly from this functionality.
tl;dr - The option to change a weapon's loaded magazine via the Arsenal menu would be very useful.