It's been a while since I did any long-distance plinking. Is an AR-15 .223 accurate at 120 yards? For long-distance I prefer my 30.06 or .308 and those are accurate to way out there. Never tried an AR-15 with a scope. I figured that the light round would lose velocity too fast for long-distance accuracy. And of course it's designed to tumble.
200yds is easy for the "AR-15". People use the "AR-15" for NRA/CMP matches, shot offhand at 200yds, sitting rapid at 200yds, rapid prone at 300 yds, and slow fire prone at 600yds.
However, usually people are using 20" barrels and heavier bullets out to 600yds, not those junk 55gr out of a 16.1" barrel.