There is a distinct lack of enemy combatants who are coming forward and saying that the 5.56 does not hurt.
As far as being able to hit a target at 500 meters, with open sites, a human sized target at five hundred meters is going to be roughly half the size of the front site post and that is standing still with full frontal profile.
Those targets rarely present themselves to shooters outside a range.
It might be worthwhile to explore issuing more optics and match grade bullets rather than trashing an entire platform that has been in the US arsenal for 50 years now.
At 500 yards 5.56 has less energy than 9mm muzzle energy, and has severe bullet drop, necessitating very accurate estimation of range in order to have your bullets not hit the ground in front of your target or sail overhead.
Yes, better optics, but also bigger, higher velocity ammo for long-range engagements.