New Army "Caliber Configuration Study"

[stanc]

I'm not so sure. A green bullet is certainly unhelpful and 6.8mm may be a better choice, but 6.5mm should still work. I ran some data through the JBM calculator to compare the ballistics of 6.5 mm 8g/123 grain (lead core) and 7g/108 grain (green) bullet with the 7.62mm M80, assuming that both bullets' form factor would be the same as that of the 5.45mm 7N6 (.929 for the G7 FF, according to the BRL), therefore resulting in G7 BCs of .271 and .237 respectively. Some of the graphs below from this article illustrate how these compare (with 5.56mm M855 thrown in for fun):

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The 6.5mm cartridge which I assumed in the above is a tad bigger than the Grendel, stretched to c.45mm.

Your comparative graphs are not only for a more powerful cartridge, you've also assumed a 20" barrel length for all calibers. The first assumption is obviously not applicable to 6.5 Grendel, and the second does not apply to a "Grendelized" M4 carbine, M27 IAR, and M249 LMG.

The question is what performance would 6.5 Grendel deliver with a 7g/108gr bullet, when fired from probable barrel lengths of 14-18 inches? Would a 65G M249 with 14" or 18" barrel come even close to matching M80 performance from the 20" barrel Mk48?