Can anyone explain to me why a .357 mag gains considerable FPS but not .357 sig?
I have a theory.
A 357 mag in a REVOLVER has a gap between the "wheel" and barrel. A bunch of hot gas is wasted blowing out the sides of the gun. A 357 sig has no gap.
So switching to a long gun not only stretches the barrel, it also eliminates the wheel and the wasted gas pressure.
Possibly, the increase for the 357 has little to do with the barrel length, and more to do with the elimination of the wheel gap.