Low erosion to prevent throat erosion?
Volume enough to keep from double charging?
Keeping from shaking the gun apart prematurely?

The slower powders than 2400 are prone to several different types of erosion.

I'd use a bulky powder between Unique and 2400 that will get the velocities that you want.

Out of what you list, Longshot looks like a good candidate.

Blue Dot may be a winner, but I don'tknow how much it can be downloaded.

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