The caliber won't change, because large scale munitions manufacturers are deeply invested in this. And because so too are today's requirements for a soldier to be able to carry enough rounds AFTER the firefight is finished, so you can return to base with at least one mag left. That's simply not going to happen with the heavier calibers.
For the most part, most nations will not change carbines. I believe there will be a small number of exceptions, and that's because IMO some will be impressed with the Tavor enough to purchase them, either for their special forces, their shock troop infantry and so on. But generally I don't think much will change, most nations will stick with what they have today.
Where things will change significantly, IMO, is:
- the optics/sighting systems: 1x to 8x, similar to but cheaper than the Elcan Spectre, increased use of thermal imager sights, possible use of miniature radar or LIDAR, electrooptical tracking/movement abilities, possible audio detection.
- barrels and receivers may be made lighter, perhaps by wider use of titanium
- muzzle devices will be improved further while keeping weight to a minimum
- designators like today's IR Lasers, may be improved and made much more advanced, with methods of thwarting enemy detection of the designator signature...unlike their ability to see that today using standard cell phone cameras.
- high speed training, perhaps inspired by the Magpul videos, and small improvements to the buttons/controls, for example the BAD levers that Magpul makes.
But generally, nations will not be interested in switching their entire military over to a 6.8mm. The USA is far too invested in this, so too is all the NATO countries, and the third world nations are FAR too dependent on supplies from the NATO countries to try some oddball cartridge that the troops may end up hating or dismissing as " uneeded". Basically, switching will cost too much, leave the troops with a heavier recoiling gun, with less ammo carried due to weight, and any shortcommings at longer distances are nearly irrelevant to nearly all infantry on a global scale, because frankly most don't have the skills to be effective marksmen at 200m anyway. And those that DO have those skills, like Canadian Infantrymen for instance, are doing just fine at 400m and 500m with no improvements. And if you really need more punch at long distances, you can do what the Brits did, and buy one of the .308 AR's for one marksman per Section or Platoon.
Keep in mind, that the vast majority of soldiers in wars, never actually end up shooting at an enemy anyway. Most are carried and only shot at the range, during times of war.
