Bob,
Rick has it covered. I have shot a 6BRX for 4 years in 600 & 1000yd benchrest. It will shoot top accuracy for benchrest at these ranges. I had over 2000 rounds on my last barrel and would still shoot in the mid 2" groups for a 4 target aggs. at 600, but in todays matches, that will get you 5th place, so I have a new one on it now. The BRX and Dasher are great rounds for long range. You may also try RL 15 in them too.
Mark Schronce