[Archie]

For maximum power and velocity (in this instance, they're about the same thing) one selects a powder according to bullet weight. Check several loading manuals and see which powder gives your selected bullet weight the most velocity.

Yes, a shorter barrel will normally produce less velocity than a longer barrel. However, the fastest load in a longer barrel will normally be the fastest load in a shorter barrel as well.

However, as has been pointed out, maximum power is not the total equation. One must be able to control the weapon in repeat shots. One must be able to reload if needed. The loading must accurate enough for the purpose at hand. One prefers the firearm serve longer than six shots. A good, solid medium power load might be preferable to a monster, maximum power load.

All the loads mentioned prior will give good service. Another 'old timer' load is a 125 grain JHP, an upper level dose of Unique and a tight crimp. (Can't remember off the top of my head, but it should be any decent load manual.) That was considered a good self-defense load in the '60s and '70s and probably still works. Unique is out of fashion, I hear, but any similar burn rate powder would do the job.