I've got a rig or two like yours and can hit gongs, 12 inch by 16 inches at 400yards and minute of man up to 600 most days, if I do my part.
As noted, wind is a big factor with 55 grain bullets.
Weapon was designed to hit minute of man out to 600 yards, some of the time, with 55 grain bullets, BUT;;;;
knock down at that distance is iffy to say the least!
Great weapon @ or under 400 yards as designed.
Past 400 yards, call in an airstrike, or arty, or get a .308 to be sure!
M-4's are not target weapons, but they can do good work if you reload and don't try and make a .308 shots with 5.56mm's.