Sticking with the 556 NATO. Ugh.
Why change? There are countless billions of rounds in circulation and millions of standard magazines. 5.56 has proven reliability and lethality. Adopting some niche caliber doesn't seem practical for the military.
The reason they stick with the mass-wound-ammo instead of one-shot-one-kill ammo is because when fighting a civilized enemy mass-wound-ammo takes out much more people and resources to care for wounded than to handle the dead.
Indicative of the anticipated enemy in a large scale ground war.