Contrary to popular opinion, the powders that yield the highest velocities in long barrels are the same powders that yield the highest velocities in short barrels. In the case of the 44 Mag that is going to be the usual list of suspects: H110, 296, 2400, N110, No. 9, Enforcer, etc.

Now, loads with these slow powders are NOT going to be very pleasent to shoot from a short barrel due to the excessive blast and flash. I would suggest that more pleasent loads at nearly full pressure levels will be much more fun to shoot and hit what you're aiming at with. They also will give you more than enough power for anything you may tackle with your 44 unless we are talking about Brown bears. They are more useful from a practical standpoint.

With your 255 gr. plain-base hard cast bullet, I would suggest you look to nearly full power loads using powders like Power Pistol, Herco, No. 5, HS-6, 800-X, or Longshot. Higher end loads with these powders (using published data) should get you 1100-1200 fps. That is more that enough. Remember, Elmer Keith did it all with a 250 cast bullet at 1150-1200 in his heavy 44 Special loads.

Good Luck and Be Safe,
Mark in GA