Optimized Caliber

[LRRPF52]

How? As far as ammo capacity is concerned, I saw nothing in the video to indicate they would've been handicapped by being armed with 6.5-7mm carbines with magazines of 25-rd capacity.

One of the biggest things on small unit leader's minds is how much ammunition they have among the Team, Squad, or Platoon. You may not have picked it up, but you can hear one Joe give his ACE report as only having 100rds, and they were barely into the fight. If you have pushed the FLOT (Forward Line of Own Troops) to the extent that there is a vulnerable gap between you and your supporting elements to the rear, if any, you start to become hyper-aware of your unit's ammo status, believe me.

You don't want to be left out flappin' in the wind like sitting ducks, so this soldier and small unit-level logistics constraint is a very real factor for dismounts. Having a larger caliber carbine or rifle would guarantee the following:

1) Less ammo capacity per space/weight
2) Less maneuverability
3) Less capacity/willingness to pursue the fight according to logistics limitations

Handicapped

You can also see how having a different caliber for all the riflemen would be quite meaningless, and how important organic indirect assets are, like the grenade launchers. It is very difficult to get into a good firing position in these scenarios, since that will become a bullet magnet really quick. They were discussing a squad attack battle drill, where one element maneuvers as the other lays a base of fire, but there seemed to be a lot of hesitation, when it needs to be executed with little or no specific instructions.

5.56 = longevity to press the fight, since there is more capacity to carry ammo. Could this unit have been well-served with DMR's? Probably. Maybe they had them, and we just didn't see it. Either way, they would not have the endurance to press the fight if they had limited capacity. Carry a basic load of and intermediate cartridge, then carry one in 5.56, and it will start to make sense as you make initial contact, then maneuver to the next position, then do it again, then check your status to see you have 3 mags left, which is time to slow down and think about where you are, where your support is, etc.

Try that with riflemen carrying 7.62 NATO rifles and they are spent really quick. Same goes for AK's, although you can carry a little bit more ammo with the AK compared to 7.62 NATO battle rifles. Now a DMR tasked to provide discreet support with well-aimed fire at targets of opportunity is one thing, your Joes with carbines need to press the fight as these guys were. This is why I like the 8x30rd magazines bare minimum basic load for Infantry, although I personally carried more without issue.

To match that load with 7.62, you would need 12x20rd mags, which is not feasible with many genotypes other than 6ft+ Northern Germanic descendents of Vikings, Slavs, Pacific Islanders, and other large-statured, high muscle-density men with a warrior spirit.