Unlikely to happen.
Current rounds are used not only by the US, but other NATO nations, and some non-NATO as well.
This means ammo can be acquired from many sources and logistical sources overlapped when needed.
Weight is also a big concern. A fast .22 like the 5.56 weighs far less than most other rounds while still doing a decent job.
This means each marine can carry more ammo and fire more rounds at a target. Since they will have to carry their own ammo on foot for miles in some cases prior to a firefight this can be very important.
Since most of the weight of a cartridge comes from the projectile, and powder is relatively light, the ideal cartridge would be one with a light projectile, and a lot of powder to make it work.
That 5.56 does that.
I think a 7.62x51 is better for many things, but it cuts how many rounds a soldier can carry in half being twice as heavy.
A shorter 7.62 round would still be heavy, just with less light weight powder (and slightly less brass to contain it).
You also arm a military not just with what will be best for asymmetrical warfare, but what will work well when they deploy against a modern military force.
Most military forces of the world wear body armor.
A lot of 5.56 pinning down the enemy while artillery or airstrikes are in bound makes more sense than running out of 7.62, or going for pure accuracy and not keeping the enemy pinned.
There is also the issue of how many rounds are realistically held in a reliable standard magazine. The difference between 20 and 30 can be huge.
When moving as a fire team a lot of moderately accurate rounds fired perform better than fewer highly accurate rounds fired.
While a skilled lone individual is better with highly accurate fire, a fire team has other members maneuvering. So it is less important that each round takes out the target, and more important that rounds keep the enemy from firing on your team. The longer the enemy is pinned the more angles he will receive fire from.
So time is against him as long as he has lead pouring down on him.
Whether he has more soldiers moving in on him, or heavier firepower being directed to his position, he is in trouble if he cannot move.
A lot of small rounds accomplish this better than fewer large rounds.
For the cartridge of a fire team it is well suited. Since they also have many larger weapons for special purpose, from AT weapons with a variety of warheads, support vehicles, .50s, automatic grenade launchers, hand and rifle grenades, etc they don't need the rifle to do everything. They need it to be an effective fire team weapon. The 5.56 and quantity of ammo it allows is perfect for that role.
It is just a rifle (and SAW), not the only tool in their toolbox.
