Pick a powder that gives you 90% fill ratio for a book max load, start 5% below max work from there, 4350, 4831 something about that burn rate, shoot it and see how it does. however you do not need a partition for deer and black bear in a 260, the cheaper cup/core bullets will be fine. look into the 140 ballistic tip, they shoot well and they perform fine at the 260's velocities.