I thought about that. The problem is a 458 SOCOM cartridge won't fit in a 5.56 Magpul mag.
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I've been able to get some in the magpul mags. But they say to use the metal ones due to the potential to bend or something like that. And if it wasn't for the 458 and 6.8 I would have got rid of all my metal mags long ago.
They should mark them for 6 rnds of 6.8 SPC. Can't seem to get more than that in them :)