Have tried it in .300WSM. Nothing magical in my loads. Have not tried it in the 6.5x55. Try some Norma MRP if you want to try something different. Accurate and fast. Bet it would also work in the .260 but H4350 works so well there is no reason to change.Just down the page a bit it was talked about-other hodgdon powders
It's showed promise in my sons 22-250ai under 70 berger vlds, in forming loads.
I also have a friend that swears by the stuff for his 6.5 creed under 140s of some sort.
I have a Savage action with a Shilen 8 twist 22-250 and a Savage model 11 in 243 and they both love the Super P. I shoot 75 amax in the first a and 105's in the second. The 22-250 shoots 3400 FPS with no signs of over pressure and the 243 shoots 3000 FPS. Accuracy has been good.