With how 400rd high caps work, you wouldn't really be able to consistently get 20rds. Ideally for the application, a magazine with an extended follower would be ideal as you can load the exact number of rounds and it will feed every one, none wasted.
And probably the people that run the closest to "real caps" are those that run GBBRs. And are generally Milsim oriented, but not always.
I do like how ammo and magazine limits make you think about ammo conservation and thinking through your shots. Makes for a more unique play experience, but generally the closest you get are games that require midcap magazines.