Both Military Technology and Popular Science had articles on it back when Samsung Techwin first announced that it was developing the SGR-A1. Given the Popular Science article was more about the technology of the robot rather than the weapon system, but both made mentions of it. The original conception of the SGR-A1 and even the Super aEgis II used a high capacity box magazine weapon rather than a belt fed weapon as it helped filter out other issues in the design before adding the issues of a belt to the design.
Due to the weight of the high capacity magazines, they were deemed pretty much unusable by anything but a stationary line of fire turret (5,000 5.56mm rounds is hovering at the 75 kg. mark before adding in the 9 kg. weight of the magazine).
Neither design that implemented the drum was ever fielded. Belt fed weapons are much easier to use for militaries, as they allow for quicker reloading and are more manageable. Then again, combat drones are near as prevalent in real life as they are in SR either.
Honestly, if you can envision a metal box holding a group of magazines (or clips if you prefer that term), with a selector at the top that switches to the next magazine each time one runs dry, you're there.
Looking at your rule suggestion, you're pretty much doing the same thing, except you're making it take an action to switch chutes (or magazines if that picture is easier) rather than letting the device do it automatically.
While manually reloading by the drone, magazine by magazine, may be a feasible design for something like the PROTECTOR super light drones, where the gun is pretty much installed in a harness without removing any hardware, I would hazard a guess that most drone guns would remove enough hardware to reload, and remove spent cartridges in branch cases, while firing.
Add in that in the past, it has worked in this way where the mounts ammunition can be freely used by the weapon (Rigger 3 and Arsenal ammunition bins), and that the tech would actually take up more room by reloading inside the mount as opposed to a forced loading system, I'm pretty sure it's intentional for the weapon to be able to feed from it's 250 round supply without issue.