You might consider trying some of Accurate's Magpro in place of your old H-450. According to the burn rate charts, it is just slightly slower burning and is a spherical powder and should meter well. I have a pound of it to try in my .270WSM, but it also lists applications for the regular .270 Win., the .243, and magnum calibers. I would guess it should work fine in the 6.5x55. I will have to give it a try in my 6.5x55 Howa. It probably wouldn't give decent velocity with any of the bullets lighter than 140 gr., but for the heavier bullets, it might work out fine.