I've shot a M4 out to 900m (APRA range) - okay I was aiming up a tree to gain enough elevation at a repetitive setting to get hits on a 4x4 screen.
Fair amount of people shoot M4's in service rifle out to 500m.
If you concerned about terminal effects - the M4 bbl will do beeter for both velocity and be able to use heavier (longer) bullets which work better too.
I have no use for a 1:9 barrel.
Fixed CH is not really desireable either.
Get the CMMG