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Length of throat us fixed in any barrel once the chamber and leade are cut, and a longer jump is known to often produce less accuracy.
The major difference between 556 and 223 is the length of the leade.
It is left longer in 556 to try and limit peak pressure while allowing higher muzzle velocity (the same reason Weatherby used it).
The AR platform is not usually accurate enough to show a great effect from longer jump.
The brass is nominally the exact same dimension, and 556 brass is not appreciably thicker than commercial 223 across the board (unlike with 308).
It is enough of a change that firing actual 556 ammo in 223 chambers is not recommended.
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