All I have ever shot at 600 is 80 grain Sierra Matchkings.
I've been struggling with this for almost a year, shooting good scores up close at 200 and 300 yards with 69 grain Sierra Matchkings then watching my score crash with some lousy groups at 600 yards with the 80s.
I'm not happy when I shoot lousy scores. The sensation was that I was shooting very well, but the 80s were never where you thought they should be. I would break a shot I thought would be in the middle and it would be a 4:00 8 ring. Then I would break the next shot down the middle and it would be a 9 at 2:00. A couple of times I shot donut shaped groups AROUND the 10 ring. Very frustrating.
Today, I shot some Lapua 77 grain bullets that we got as samples on Commercial Row on Camp Perry at the National Matches. I only had 20, and the match calls for 2 sighters and then 20 rounds for record, so I shot two Sierra Matchking 77 grain bullets for sighters. The sighters were a 10 and an X. First Lapua 77 just missed the nine ring for an 8 at 12:00. I came down three clicks on the rear sight and started shooting up the center. Result: 194X8 (out of a possible 200) Everything on call. Much fun when things work.
Overall 485X16. 95X1 standing at 200, 97X3 sitting rapid at 200 yards, 99X4 at 300. Beautiful day at Panola.
Update: Yeah, if I had a 1/7 twist barrel there wouldn't be a problem. Same thing with my 1/8 twist match rifle barrel- it's 24 inches long instead of 20 on the AR service rifle, so it seems to shoot 80s OK. Those extra four inches or more twist rate make a big difference.