I just posted a tool for helping folks estimate muzzle velocities when their loads involve different barrel lengths and reduced loads compared to existing load data:
http://shootersnotes.com/calculator/velocity-estimator/
The estimator is effectively a modest extension of the Powley calculator. It allows you to use your powder even though it might not be one of the IMR powders the Powley calculator was built to handle.
The method was calibrated with jacketed bullets. Folks have, however, pointed out that it may work with cast bullets and, possibly, black powder. I am curious about how well the method might extend into this arena.
To use the tool, you will need to enter a reference load that uses the same powder but with a charge weight equal to or greater than the one you use in your load. Same for the bullet, it will need to be the same weight or heavier than the one you use and more likely even better if its the same brand. Barrel length for reference load is needed along with your barrel length.
Let me know how it goes!
Thanks!