I've got a Sega Genesis...
I was making some tweaks to the model tonight and rendered up 6, 8, 10, and 12 chamber versions. Past 14 I seem to have a bug in my code which gives the compiler grief.
Attachment 88459
8 rounds looks manageable. At 10 and up, the single diverter geometry looks broken. The space the round needs to traverse appears too large. It can probably be fixed with multiple diverters.
Personally, I'm going to stick with 6-8 rounders. Larger than that and you're paying a penalty for all that dead space in the middle as well as more "dead zone". And the torsion spring probably needs to change. All that extra space in the middle does give me another idea though...
An alternate design if you like huge drum mags is something like this (not mine):
https://www.youtube.com/watch?v=Z4LY5nMvrZg
It's not really a rotary loader; it's more like a curved conventional magazine. Still all the same faults as a typical magazine -- everything has to be pushed from the back by a single pusher, and you can't top up the drum while it's in use. But something like that might be up your alley. Massive "dead zone". And... probably going to be pretty expensive to 3d print, unless you own your printer.