9mm velocites are not effected much by barrel length. I've generally seen about a 100 fps difference between a 3.5" barrel and a 5" barrel. The modern bullet designs of today have a much wider upset velocity range then the difference you will see in fps from using a shorter barrel. Mind you this is just talking about 9mm. Other calibers have their own characteristics.
Well that's good to know. It's been a while since I looked at velocity in relation to barrel length and darned if I know anymore how fast they need to go to be effective. I like to carry my glock 26 as a BUG to my g17 or sometimes just carry the 26 with two extra G17 mags so it's nice to know I can run the same ammo through both without much impact on terminal performance.