Bigger punch. Against rag tags, 5.56 is fine. Against well protected troops, you need to up the ante.
That isn't really true.
7.62x51mm is, like 5.56x45,, or 5.45x39mm, 7.62x39mm, 7.62x54mmR the case of the ammo self!
These caliber dicussions are annoying.
It depends on the weapon, the barrel length, the ammo, the doctrine.
It's actually funny that today the 5.56mm is being attacked because of the alleged lack of man-stopping effect, and at the time of the Vietnam War it was openly discussed whether the ammunition, because of its extreme man-stopping effect (extreme injuries), would not violate the Geneva Convention (Munition-type). The man-stopping effect of a 5.56mm NATO Standard SS109 from an M16 on 300m is greater than that of a 7.62mm NATO Standard from a G3. The man-stopping effect of a 5.56mm NATO Standard SS109 from a G36 is greater than that of the same caliber with that of the same cartridge from an M4. The man-stopping effect of a 5.56mm NATO Standard SS109 from an M16 is up to 400m greater than that of a 7.62x39mm full jacket from an AK. The same caliber and the same cartridge from an M4 but have a lower man-stopping effect than the AK. The projectile's susceptibility to wind is significantly lower from an M16 than from an M4 and less than from a G36. Nevertheless, (nearly) everyone who could choose would prefer an M4 to an M16 because it is more compact and more manoeuvrable. An M855A1 from an M4 has a significantly higher man-stopping effect than an M855 from an M4. An MK318 from an M27, in turn, has a much higher man-stopping effect than an M855A1 from an M4. A 5.56mm M995 or an Israeli APM penetrates on 100m body armor, which does not penetrate a standard 7.62x51mm NATO bullet at the same distance. Conversely, a newer 7,62x51mm AP ammunition penetrated plates which neither of the two cartridges mentioned can penetrate even close. etc.
The best would be completely new munition (but expensive, in every sense - allies would need too etc.).
Murricans, if they do it right this time...
6.8x51mm (Hybrid) (Better than (typical) 556/762 and the .338 Norma Magnum (8.6x63mm) (likewise hybrid munition) is a BEAST!
But as I often said, Iran and US will never wage (full-scale) war directly.