Re: And the point is?
Yes, the 5.56 round is smaller and lighter, but with vastly lower penetration capabilities - especially when over 300m.
In fact I believe that most engagements are at a rather shorter distance anyway. At 300 yards a human sized target (OK: a Figure 11) is remarkably small and aiming an effective shot requires that the target is exposed (I nearly wrote "exposes itself" but thought better of it) and immobile for long enough for a decent aim to be established and the shot released. In turn that may leave the firer at least partially exposed to return fire.
At any sort of "long range" a well concealed sniper complete with a decent telescopic sight is a much better bet for effectiveness. Suppressing fire can be effective at any range but can be an awful waste of ammunition.
0.338 Lapua anyone?