Factors that IMO should be primary:
- Short-action; preferably no longer than .223 oal, (logistics issues)
- Light- to medium-weight (physical weight; logistics as well)
- Moderate recoil (most ‘modern’ soldiers aren’t experienced shooters)
- Flat trajectory (simplifying things again for the ‘non-shooter’ soldiers)
- Decent performance on personnel armor (likely to become more common)
Caliber could probably be anywhere from 5 to 7 mm. Bullet configuration would be important for trajectory and terminal performance. Any longer cartridge such as .308, .30-06, etc, isn’t size-efficient enough for general infantry use (my opinion, obviously).
I personally prefer the 5.56, in large part because of the .22 conversion issue. This is relevant to me as an individual user, but I doubt that it’s a real big concern for the military, so the 6.8 may be a better “all-around” military choice. Only time will tell how well it works.
As far as weapon platform, a gas-trap AR configuration would be close to ideal, but some others are good as well. A major improvement I’d like to see them make is in the gun’s barrel itself. Instead of standard rifling in the barrels, go with polygonal progressive-gain rifling, as ArmsTech uses. This increases velocity, and therefore power, dramatically. For comparison, their 9-inch 5.56 barrel gets slightly more velocity than a 14 ½ inch ‘normal’ barrel on an M4. So a gun with a compact 14-16 inch barrel could offer the same velocity as a 21-24 inch “normal” barrel does now.
Use a gas-trap upper with a 16” ArmsTech style barrel, and you’d have a gun with lower maintenance requirements, compact CAR-15 handling characteristics, and varmint-gun velocity and range. Heck of a deal, regardless of caliber.