Grendel mags?

[pinzgauer]

But, I think such an extra-thick walled magazine would fit in standard mag wells. The reason I doubt it'd be viable is because the closer spacing of the cartridges might not align properly with the feed ramps, and also because most shooters wouldn't like the extra-long mag.

OK, gotcha. Make the mag walls thicker to handle the side pressure due to the narrow stack.

You'd probably be down to a 15-20 round mag to keep the same form factor as a 5.56 30 round. You'd have to really want a plastic mag badly to accept that compromise, especially when 25 round metal mags work fine.