I've been looking at that for my next build.
Deciding between 6.5 Grendel and 7.62x40.
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Personally, I'd go .300 BLK over 7.62x40. Not enough performance difference to matter in the supersonic loads, and the BLK should handle subsonic much better.
What the heck is the 7.62x40? Never heard of it.
A .30" caliber created for the AR15. https://en.wikipedia.org/wiki/7.62×40mm_Wilson_Tactical