Click on the 357 mag icon and look at the different velocities based on barrel length.
Most of the published data you see is from 8" test barrels with no cylinder gap. Any rifle or handgun chambering needs some barrel length for all of the powder to burn. Most will burn within a few inches, but the pressure still builds up as the bullet passes through the barrels. A 357 from an 18-20" rifle barrel will be quite a bit faster than from even an 8" barrel. A 4" barrrel is only 1/2 the test barrels length and a 2" is 1/4. You will lose serious speed from barrels that short. A 2" barrel in most loadings is 500 fps or so slower than from an 8" barrel based on the above link. Almost 1000 fps slower than from an 18" rifle barrel in some loadings.
Also some cartridges use less powder, or are more efficeint and don't need as much barrel to reach their full potental. Magnum handgun chamberings with barrels less than 4" long are about useless for my purposes. Most of the published data for semi-autos is more accurate because they are tested in barrels closer to what people actually use. One reason the 9mm killed off 357 mag in law enforcement was when folks figured out their 6 shot 357 mags with 4" barrels were only shooting around 50 fps faster than a 15 shot 9mm when using the common 124/125 gr loadings.