Not that I'm necessarily suggesting this, but if you want maximum velocity at safe pressures from a 6.5x55, the best powder for this is Alliant Re26. According to QuickLoad, 2900 fps is possible with this powder from a 24-in barrel with the 140 Nosler Accubond bullet at a pressure just under 60,000 psi. That is 60,000 psi by the piezo transducer method, not the older, now obsolete, copper units of pressure (CUP) method. The 60,000 psi is approximately equivalent to 52,000 CUP, and so should be completely safe in any modern 6.5x55 action. When I do the calculation using the same pressure maximum with Re22, H4831, IMR4350, Norma MRP, and Vihtavuori 560, I get to about 50 fps short of 2900 fps. Of course, you’ll want to experiment with your rifle to see which powder produces the best accuracy, but, if possible, Re26 should be among those tried.