Our military adopting the Grendel?

[stanc]

Yes, you can "only" carry 26 rounds in a Grendel mag instead of the usual 30 of the 223 rounds... True. But on the other hand, you only have to shoot the bad guy ONCE with the Grendel. This means that you actually carry 26 potential "kills" per magazine with the Grendel versus 15 kills in a 223 mag ( assuming you have to shoot them twice)...

That's the potential flaw in your argument. You are assuming it won't also be necessary to shoot them twice with 6.5 Grendel.

Then there is the matter that many (if not most) rounds fired in combat miss the intended target.

And in a protracted fight, such as experienced in the 1993 battle in Mogadishu, having 210 rounds (7x30-rd mags) vs 175 rounds (7x25-rd mags) could make the difference between staying in action and running dry.