This is not even comparable to the assassination attempt on Trump, where the guy fired multiple shots and missed. The person who killed Charlie knew what they were doing, aiming at the jugular artery, possibly a professional. The fact that they couldn't even find the person makes no sense. How long is it going to take for someone to escape the roof top and get away with it? Isn't US intelligence so good it can catch people doing crimes from different parts of country and even the world? If they don't catch the person it's because it WAS an inside job..
To get to 1moa hand loads or premium match grade ammo is basically required. Common cartridges like m855 hit around 3 or 4moa, even if your rifle and aim are flawless.
Beyond that the rifle needs to have a barrel/crown in good condition, and everything must be tight and torqued correctly. This is a common issue in my experiance since most people dont bother with a torque driver and shooting rattles things loose if you dont.
Last, and this is just my personal opinion, but the higher zoom the better on your scope. If you're at 4x everything seems stable, you put the crosshairs on the target and pull; but switch to 24x and all of a sudden the target's bobbing around like crazy and you realize you have to be much more careful than you thought.