I feel like the current system has a handful of different options that suffice fairly well. Is there another system (or alternative system) someone thinks would be better?
If the magazine is unloaded, it's a "drag-drop ammo box onto the magazine". If it's loaded in the gun, then it's you only add one step of "right-click gun, and then drag-drop ammo box onto the magazine".
Right-clicking to view the physical storage slots of magazines and ammo boxes is only necessary for players who want to carefully interact with load order for something like tracers. Alternatively, players can use the search function next to each attachment slot so they don't need to drag-and-drop at all.