I think that the ability to use curved magazines lies in the sidewall taper of the case.
the 7.62x39 has more of a angle than the 5.56x45 cartridge, thus the ak mags are more curved.
all one would need to do to use curvey mags would be to figure out a sidewall taper that fits the mags best.
also, is the idesa of this new cartridge to need zero modifications to the ar aside from a new barrel? I would love to have an upper that all I need to do to increase my firepower is to swap mags and barrels....
the difficulty with that is finding a parent case whose base would fit in the 5.56 bolt, and diameter large enough to fit a .40 cal bullet in.