I also thought I would need a fast powder for the 10 inch barrel. I wrote Sierra for advice. They told be the same thing that Mule Deer just said, slow powders for rifle cartridges, regardless of barrel length.
Same experience here. I use the same powders regardless of barrel length. For a direct comparison I shoot an Encore in 260 with a 24" and 15" barrel. Same powder gave best results in each.
