Testing, testing...

[stanc]

You could be right -- but we'll need more than a cartoon to resolve.

Now, why did I know you were going to say that?

Problem is, I was (and might still be) under the impression that there's a gap between the cartridges.

Perhaps that's due to the post by Harrison on the old forum, where he said that 6.5 Grendel rounds have a gap between them? Even if he was right, it's because of stuffing a .441" case into a mag with external dimensions originally made for a much smaller diameter round.

With mags designed for a specific cartridge, there is no gap in between rounds. I couldn't find a photograph of a sectioned magazine, but the Garand clip has its cartridges stacked in the same manner.

...if the magazine is purpose-built for the cartridge and, as you point out, the cartridges are stacked as efficiently as possible (the reason for the double contact between bases), then the magazine thickness also decreases.

That's true, and was assumed in the mag capacities I listed in my previous post.

However, capacity for mags of the same height as 7.62x51 mags would be as I showed. You can see a good example of this in the 20-rd 6.8 SPC mag marketed by Pro-Mag. It has almost the same height as a 20-rd M14 magazine (but slimmer, as noted in the previous paragraph).