Lee.
I have 6br and 6BRX in all the tw, but a 9. I have a 10 tw that shoots the 90BT Berger and 95 Bib's great. I have shot 600 with 80gr Fowler's in a 12tw 6BR and done very well. But All the IBS records but 3 have been set with 105 Berger. I have lost count, but it is 30 some records have been set by the 105 Vld Berger. All of the current records, but one, are held by the 105 Berger.
I have seen 68 gr bullets do well in good conditions, but if the wind gets up it as half the BC of the 105.
I had a 10tw 6BRBS (6BR IMP) that would shoot the 95 VLD very well out to 1000 yds.
If I were setting up a 600 yd gun in 10tw, I would set it up for the 95 VLD Berger. They have very little bearing surface, and would need a short freebore, like for light bullets, not for the longer 105. My 10tw have the longer freebore, I can't touch the landings with the 95 Vld's.
If the gun is set up for the 95, then you are set for any bullets from 60 gr to 95gr bullets.
The best thing for 600 and 1000 yds is the 6BR IMP's (6BRX, 6 Dasher,etc.). The 10tw you were talking about was a 6BRDX shooting 88 Breger's , and the record was small LG group 1.02xx" in 2005". That record was broken in 2007by a 6BRX shooting 105 Breger 0.749", then it was broken also by a 6BRX shooting 105 in 2008 0.711" by Rodney's son, Paul.
A 10tw would work fine for 600. The 95 Bib is the best bullet in my 10 tw.
Mark Schronce