here is a spanner thrown in. drop both and adopt 6.5 Grendel!
Per Wikipedia, only the Serbian Army is using 6.5 Grendel.
Good luck getting your hands on stockpiles of 6.5 ammo in the heat of battle. 5.56/7.62 NATO and 7.62x39mm, on the other hand, are readily available.
For example, Japanese Army rechambered their Type 99 rifles to shoot .03-06 Springfield round back in WWII due to the scarcity of 7.7mm rounds. Meanwhile, they'd gobs of .03-06 ammo crates, captured from U.S army.
5.56 is a very small round for the ranges that it needs to be utilized at---.
Exactly. At close ranges you need kinetic energy, not velocity. And while the low recoil of the 5.56 does offer superior control while on full auto, soldiers rarely go Rambo!
When they 'do' shoot on full auto, it's usually to lay down suppressive fire, no accuracy needed. You just need to spew a barrage of lead in the general vicinity of the enemy.
I have a question, why haven't we fielded the AK-74s instead of AK-47, it has lighter recoil, flatter trajectory, better wound characteristics and lighter ammo means more capacity for a soldier to carry
Trajectory can be easily compensated for, and the notion of 'better wound characteristics' remains highly debatable. While 5.56/5.45 bullets have a tendency to yaw in soft tissue, it is not 100% guranteed. More often than not, the bullet will connect with a bone and merely incapacitate the target, unless you go directly for the belly.
Regarding velocity, the 5.56 does have an edge with its "theoretical" potential for hydrostatic shock, but the 7.62x39 velocity isn't all that far behind (730 vs. 961 m/s).
Plus, armies usually use 123 grain bullets when it comes to 7.26x39, which are basically twice as heavy as U.S Army's standard issue 62 grain 5.56 ammo.