The skill set may need to have the rust knocked off, but I'm sure you can still hit those 600 yd. targets with an AR.I shot out to 600 yds accurately with an M16A2 back in the day shooting the Wilson matches at Camp Robinson in Little Rock, AR. I don't believe I can do that anymore, but other than maybe an NRA Highpower match at Camp Atterbury (they still do that?), I don't know where I could find out.
You can find out right here ... CIHPRS - CIHPRS Home