How could or should the 6.5 mm Grendel be improved?

[stanc]

As Stan C points out, the difference in a 5.56 mm magazine is about 25 rounds of Grendel versus 28 rounds of 6.8 Rem.

No, no, no, no, no! The actual difference is 25 rounds of 6.5 Grendel versus 26 rounds of 6.8 SPC, tops! At best, that's one round advantage to the smaller case diameter, not three!

This is easily calculated. A 25-round, staggered column magazine has one column of 12 rounds, and a second column of 13 rounds.

13 x 0.44 = 5.72 inches
13 x 0.42 = 5.46 inches

Difference = 0.26-inch, which is less than the 0.42-inch diameter of the 6.8 SPC case. Maybe sufficient for 26 rounds capacity, but not 28 rounds.