How could or should the 6.5 mm Grendel be improved?

[stanc]

...outside the wire in a combat arena, I want general purpose rifle (GPR). The problem is that as a GPR, the M4 (5.56x45) is too light and the M14/M110 (7.62x51) is too heavy. A GPR using a 6.5mm round makes more sense.

In theory, I agree. But, I'd sure like to see some comparison testing done to find out if theory translates to reality. Say, perhaps have a number of shooters with varied skill levels, run through different scenarios alternately with a 5.56 M4, a 6.5 Tac 14.5, and a 6.5 Tac 16.