I have many 357 magnums. my 3 favroits are my Coonan Classic, My Marlin 1894 trapper and my S&W 640.
Bullet speed varied alot in a 357 magnum based on the barrel.
As far as where barrel starts to slow down a bullet rather than speed it up? I would guess its some where between 16 and 20 inches because my 16 inch trapper shoots faster than my oldest sons 20 inch marlin and my 16 inch marlin shoots quite a bit faster than my 6 inch Coonan hunter barrel.
Shot placement is everything! I would rather take a round of 50BMG to the foot than a 22short to the base of the skull.
all 23 of my guns are 45/70 govt 357 mag, 22 or 12 ga... I believe in keeping it simple