Kadagar:
Each round represents different concepts/preference.
.30 rounds have a great deal of impact and are designed to kill a human target at range and to knock down any target hit even without a kill. Man-stopper. Assault rifles less so than old-style battle rifles, but still the emphasis is on stopping power.
.223 rounds do not create the same impact on a target, and were originally designed as a "wounding" weapon that would force an enemy to expend resources removing wounded warriors from a battlefield rather than gathering the ammo of a dead comrade and pushing forward.
Both will penetrate cinderblock at combat distances. The 7.62's mass is more destructive, but a human on the other side of the cinderblock still gets hit. Both will punch through small widths of steel. Functionally, penetration with standard rounds is probably a wash. The only real point of advantage for the 5.56 is against soft body armor, where its high speed and narrow hitting area allow it to exceed the tensile strength of the armor fibers slightly more effectively than the 7.62.
For those fjord-fights you allude to, where precision at distance would be of value, the real issues would be barrel length, bullet stability, and optics. A Garand might be better than any assault rifle in such conditions.
Logistically, the 5.56 allows a soldier to carry significantly more ammo and to fire a weapon for which the vast majority of the recoil can be cancelled out. Since most bullets in combat are used as suppressive fire, and a typical human is no less likely to duck when shot at with a 5.56 instead of a 7.62, more ammo is generally a useful idea.
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