Who decided that 30 rounds is optimum?

[Secondgunman]

Weight of the rounds, magazine spring tension, standard ammo packaging ie 3/20 fills 2mags. The sten. Sterling austen used side mounting in part for reliability not having to overcome gravity to feed. Basically why are martinis like a woman's. Breasts , cause one isn't enough, and three is too many

Yah this is about the just of it...