Grendel as a Universal Infantry Cartridge

[stanc]

And, yes, staggering the stacks does indeed complicate assessments of magazine length, numbers of cartridges, and so on.

I did not say that. I see no such complication. For a staggered double column, double feed magazine containing an even number of rounds, total stack height will be the height of one stack plus a hair more than half the diameter of one cartridge. It's really pretty simple. Below is a drawing showing this. (Since this pic is of a single feed mag, disregard the positioning of the top four rounds.)

Another way interpreting the stack height and magazine thickness might then be that the differences in dimensions of a 30 round magazine will be effectively negligible.

Yes, I agree. Where the difference would be noticeable is with a 30-rd 7.62x51 mag vs the 30-rd GPC mags.