By the way, Sam, here is some good reading on the subject by Lee Jurras:
http://www.leejurras.com/barrellength.htm
Note he did what WESHOOT2 mentioned, he sawed down the same barrel instead of trying to extrapolate what loads would do in guns with different barrel lengths. Even then, you'll see that some rounds got higher velocities out of a shorter barrel. Lot's of variables at work there.