Snub Nose Velocity Test - video
Here is a video that I found on youtube.
Knowing that most published ballistic data for .38/.357 loads are using a 4" barrel, this guy set out to get an average reduction using a 2" snub.
Here are some of his findings, using a variety of ammo. This is an average, some of the ammo performed better than others.
Again, the loss percentage is comparing a 2" barrel to a 4" barrel.
Overall the % loss in energy with the SP-101 was 27.7%
Overall the % loss in energy with the Model 442 was 26.2%
Check out his comments on the youtube page.