How Does Barrel Length Affect Overall Power?

[Chickenthief]

Yes and no.

Thats a good answer right ;-)
It depends, is a more appropriate answer.
If the powder used is fast enough and the barrel long enough it ends up slowing the bullet down.

Take a look here:
http://www.ballisticsbytheinch.com/9luger.html

See at 18" the speed has dropped.