Most published 357 mag ballistics are from 7.5"- 8" test barrels. Firing them in 6",4",or 3" barrels would be equivalent to shooting a 300WM from 18",12" and 9" barrels. The velocity numbers are going to be considerably slower, especially with barrels under 4".
Almost all semi auto chamberings show published ballistics from 4", sometimes 5" barrels. This is much closer to the barrel lengths people actually use and velocities will be much closer to what is published. My 9mm G-19 with a 4" barrel, matches 357 loads from a 3" barrel with 124/125gr bullets. I'm getting 1250 fps, same as the Ruger 357 in the previous post with my G-19.
Some good info here.
http://www.ballisticsbytheinch.com/index.html