After multiple powders and several bullets, 2 bullet types and loads have provided the best accuracy for long range in my 6.5/300 RC Accumark.
I did rebed the action and use a Nightforce NSX 5x22x optic.
Hornady 143 grn ELD-X
US869- 89grns , C.O.L 3.610
Velocity Av 3250
Berger 140 Hunter Elite
US869-89grns, C.O.L 3.610
Velocity Av 3288
Note: testing was at 3400 Alt
3 shot cold bore groups provided best results. 4,5 shot repetive groups showed slight signs of heating, resulting in climbing groups.