Vihtavuori powder is some of the best. Perhaps because it's made in Finland, its very temperature resistant--and as of 2-3 years ago, it all includes a decoppering agent.
The 100-series powders are single-based, and result in longer barrel life. The 500-series a double-based and result in somewhat higher velocities. So much of the choice depends on how often you shoot a particular rifle.
In the Grendel I found N540 worked very well with heavier bullets. In the 6.5x55 you might try their relatively new N555, which was partially introduced for use in the 6.5 Creedmoor, which has a similar case capacity.
I have been wanting to try the N555 for awhile now for my 6.5x55, just haven't seen any in various MT locations and online. Have you seen any in MT?
