Are you sure?
Looking in the Editor in APEX it "appears" to be pretty straight forward that it is supposed to work like that. You drag the weapon onto the ground (or where ever) and in the options for that item there is the weapon and ammo listed at the bottom with + and - signs to add more or subtract. Doing this will make the weapon disappear yet leave the item box on the screen. You have to delete it and place the weapon again to get it back.
I am far from an Editor expert but it does seem to me that it is the intended use.
Thanks
Here's a quick vid showing what I am attempting...
EDIT: This seems to work reasonably well if still interested.