I gotta share this website with you... here these guys have done some really impressive testing to demonstrate how bullet velocity is affected by barrel length...
Last year I was developing a load for my 4" .357 Magnum and couldn't understand why I wasn't getting 1500 fps from my loads like they did from the load book... then i didn't really understand very well how a longer barrel increases bullet velocity, all other factors being equal... anyway, check out this website here:
Ballistics by the Inch
and they share their data for free here to download:
after reviewing their data, i soon understood why i would never get 1500 fps out of my 4" bbl and have the gun remain in one piece, and going for a target velocity of 1250 fps was more realistic...