The .223/5.56 is adequate out to 300 yards. Anything beyond that and retained energy is questionable. The effectiveness of any round at any range is the amount of energy that you can put on and in a target. For extended range shooting a heavier bullet traveling and a good velocity is a much better choice. It has and retains superior energy to the smaller lighter bullets. Hyper velocity lighter bullets do not make up for the difference in weight. That is not how the math works out.
I would take a look at the Shepherd scopes. I have a couple and use them on long range targets/hunting. The key to this scope is you have to reload. By being able to adjust the velocity you can fine tune the down range bullet impact and really get great results at 500-1000 yards. If you reload and do the research and practice this scope makes long range shooting much easier for most.
I think with advent of more reliable frangible projectiles, the lethality of the 5.56 mm round extends to 400-600 yards depending on barrel length. It has always been a bullet full of compromises which makes it a jack of all trades/master of none kind of round. Despite that, from 0-300 yards, the 5.56 mm is very capable of creating devastating unsurvivable wounds. The low recoil allows one to create many such wounds in multiple targets in a matter of seconds. But if I knew that all my targets were going to be 400 yards and beyond, I would definitely be looking at a .308 winchester or better. I’m familiar with shepard scopes. I’ll have to consider that option. I like fine crosshairs but there’s no denying the shepard’s useful features.