The barrel length thing is a rule of thumb in a way. While it is true that longer barrels will usually produce more velocity, its not true in every case. Some calibers do not benifit from more barrel length and some do. A few years back a gunsmith friend and I bought about 25 6.5x 55 Swedish Mausers. They all came with 29 inch barrels. We sporterized and sold them in my shop. We cut them all down but on many we cut them off one inch at a time and chronographed them with the same load. Some lost velocity as we shortened the barrels. A few gained velocity and a few didn't change at all. Most of them lost velocity if we went below 22 inches of barrel so we cut most of them to 24 inches and left it at that. We didn't take time to work up different loads for the shorter barreled guns to try to gain back lost velocity but powders of different burning rates can do just that. The gun makers have set their barrel lenths at 22 inches for standard calibers and 24-26 inch barrels on Magnum models figuring that the longer barrels will burn the larger powder loads better. For the most part this holds true. Most of the reloading manuals use barrels of 24-26 inches for their test data which tells me that that would be about the best length to perform to their max.

In the real world of the average hunter the barrel length and velocity are probably not as important as a consistant load which produces the best accuracy in your gun. A shot through the vitals is far more important that the speed it arrived at and eeking out the last FTLB of ME. I try to arrive at that load then mess with it some more if I have the time or need.