I like Pmags for the 5.56, but the 6.5 Grendel really benefits from the additional OAL capability of something like ASC's stainless mags. Pmags are pretty limiting in that regard, to the extent that I wouldn't buy any for my Grendel use.
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With that being said, a lot comes down to numbers. 5.56 is a no-brainer, but every product we release has a robust business case supporting it. 6.5 Grendel presents some engineering challenges (as mentioned above) and the market size is a big factor.