Normal mags: max 7 mags of 30 rounds.
Extended mags: max 5 mags of 40 rounds each.
Drum mags: max 3 mags of 60(?) rounds.
*actual/better numbers should apply.
With this set-up drum mags become a real question of "how often do I want to reload?" instead of "Do I want more ammo? Yes I do." Kind of like removing AP rounds this would make drum mags a non-obvious choice.