Either it's a minor dev oversight, or it's working as intended. While wielding an SMG, you can't replace it's equipped magazine by doubleclicking another magazine in your hotbar. You'll simply switch to equipping the doubleclicked magazine to your hands.
Performing the same actions on a rapid fissile accelerator, everything works as expected: doubleclicking or pressing the equip indicator above a fuel rod in your inventory will put it inside the RFA and swap out whatever was in the RFA before.
Unless, the RFA has nothing loaded into it, in which case clicking the equip indicator above a fuel rod will not load it. Doubleclick works as expected in that situation. That might be a second oversight.
SMG can equip a magazine via doubleclick but not by clicking the equip indicator, if the smg's internal slot is empty of any item.
Either it's a minor dev oversight, or it's working as intended. While wielding an SMG, you can't replace it's equipped magazine by doubleclicking another magazine in your hotbar. You'll simply switch to equipping the doubleclicked magazine to your hands.
Performing the same actions on a rapid fissile accelerator, everything works as expected: doubleclicking or pressing the equip indicator above a fuel rod in your inventory will put it inside the RFA and swap out whatever was in the RFA before.
Unless, the RFA has nothing loaded into it, in which case clicking the equip indicator above a fuel rod will not load it. Doubleclick works as expected in that situation. That might be a second oversight.
SMG can equip a magazine via doubleclick but not by clicking the equip indicator, if the smg's internal slot is empty of any item.