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So going from 20 to 24 inches gives you 33 FPS increase? According to online ballistics calculators, 16 to 20 inches will give an increase of 30 FPS (depending on many factors). Some calculators have the FPS decreasing (again lot of factors).
I would not want an extra 4 inches of barrel just to get 1.5-2% increase in velocity (2050 vs 2080) for a gun that's meant to be carried. I'd go under 16 inches in 357 magnum if SBR / NFA laws didn't stipulate that.
Edit. In fact I did. I forgot. I have an AR15 variant in 357 magnum, it sports a 12 inch barrel.
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Yes, Ballistics by the Inch shows that, however, and it's a big however, they are using commercial pistol ammo specifically developed for pistol length barrels (4"- 6"). If you look at Buffalo Bore's 357mag Heavy ammo or even better, handloaded ammo using Magnum pistol powders like H110/296 or Lil'Gun, you'll see that they perform significantly better in longer barreled guns.
My normal, full power 158grn 357mag loads using SAAMI spec 16.5grns of H110 develop 1,382fps in a 7.5" barreled SA, 1,789fps in my 20" Rossi carbine, and 1,822fps in my 24" Rossi rifle. That's 407fps better in the 20" carbine vs the 7.5" pistol and 440fps more in the 24" rifle definitely showing that the longer, 24" barrel isn't slowing the bullet down as they suggest but rather still increasing the velocities. Remember, I recommended that the OP get the 20" carbine over the 16" one, not the 24" rifle.
Further, I didn't get the rifle for 33fps more velocity. I got it so that I could get a 30" sight radius by adding a tang and globe set of sights vs the 16" sight radius from the sights on my carbine. Why? I use my rifle length Rossi with my Zero 158grn JSP bullets and H110 powder loads to shoot at steel plates at 300yds . . . . . . . . . iron sights and a pistol bullet. That's why.