The rounds on the GAU-8 and the M-61 area also arranged as a helical spiral with the rounds arranged radially, point in. That arrangement isn't gonna work on a 57mm or 75mm round unless you want a 5 or 6 foot diameter pod. They are gonna have to be arranged along the same axis of the gun, limiting the ability to store a lot of ammo unless there is another mechanism that advances ammo from a rear carousel to the front one, not impossible, just more complicated and probably requiring additional power to drive it versus using just gun recoil.
Paul