There is the quad stack magazine, found on that of the M4 Spectre and the AK series. Theres also the new gun powered magazine found on the LSAT rifle. Combining the two, I think you can have a mag that can hold 32 rounds, or even 60 like the AK quad mag, requiring very little space.
Oh and BTW, the 7.62 NATO are rimless.
Oh and BTW, the 7.62 NATO are rimless.