Alliant Power Pro 300-MP

Havent used it in carbines yet, but I love the powder. I use it in all my magnum revolver cartridges. I have found it to give excellent velocity and accuracy. I haven't found a reason to use h110 or w296 since 300mp came out.

In my experience in a 6" Ruger GP-100, it is very similar to W-296/H-110. It did provide an additional 50-75 fps and requires a slightly heavier charge of powder to get there. If you are looking for maximum velocity, particularly out of a longer barrel, Power Pro 300-MP should be the first one to try. If it meets your accuracy requirements, you are probably done. (It was very accurate in my revolver, YMMV)
Note: Alliant's published loading data was obtained using a 10" barrel and is quite optimistic, if you are loading for a conventional length revolver. Not sure how much more velocity would be gained by a carbine length barrel.