Following a lot of brilliant help I got on this thread: What 260 (6.5mm) bullets are you having success with?
I settled on N160 as the powder of choice for me - throws Nosler 120 BTs through the same hole and Accubond 130s under half an inch when I do my bit. Had the chance to run the loads through the chrono last night though and a bit surprised by the results, much slower than I was expecting:
42.3gn N160, 120gn BTs - 2611 , 2588, 2641 - average 2,613fps
40.8gn N160, 130gn Accubond - 2555, 2536, 2532 - average 2,541fps
Those are definitely the sweet spot loads as 0.3gn above and below and the groups start to open up.
As per the other thread, I've tried a load of powders now (N160, H414, H4350, R17), 3 different primers and multiple jumps to lands, and the loads I've settled on seem by far the most accurate, but was expecting velocities in the 2700 - 2900fps for bullets in the 120-130gn range. Am I chasing unnecessarily velocity by messing around further??
Edit - barrel is 21" pacnor