Testing, testing...

[JASmith]

...They made the mistake of not going quite big enough. 6mm is the minimum necessary for long range, 6.35mm might've been the best, considering the trade-offs for minimum cartridge size and weight, and acceptable ballistics.

You're probably closer to the mark than most.

At the risk of re-plowing some ground, my thinking is that the 7.62X51 NATO will almost certainly continue to be the "light caliber" for vehicle-mounted weapons. I think it may be possible to go with a unified caliber for dismounted units, but sense that the emerging plastic-cased ammunition Tony has observed will keep the 7.62 in the heavier dismounted units as well.

This opens the door a crack for the intermediate caliber by decreasing the high-end requirement. It would truly be interesting to see what one could get from a family of Grendel-based cartridges in 5.56, 6, 6.35, and 6.5 mm. We should also examine a similar family based on the 6.8 SPC.

LR1955 and others have argued that probability of hit out to 400-600 meters is of critical importance and that any new system needs to shine in at least one area.

Given the fundamental need to get a hit, it seems odd that we've spent a lot of time discussing on-target effects, ammunition weight, and recoil but almost none on trajectory and wind-drift. These areas are ones where higher-velocity and moderately well-shaped bullets might result in a significant improvement.