H1000 may be ok for the heavier bullets but there are better powders for overall use. Most of the 4350 family and even 4831 family work well. Alliant Rel-19 is superb, as well as Rel-22.
I have a 6.5x55, and it's a near twin of the 260, and I use H414 with the lighter 100gr bullets, and Rel-19 and 22 with 140gr.
NCsmitty