The key to understanding short barrel ammo is that 1) bullet design is optimized to the velocities one will have from 2"-4" barreled guns, and 2) that the recoil from the round will be at least tolerable in a lightweight carry gun.
The Speer 135-gr. bullet expands reliably from about 850-1050 fps. Below that, minimal expansion; above that likely fragmentation. In my tests with it, the 38+P runs about 880-880 from a 2" barrel, and the 357, about 970 fps from the same revolver (M&P340, 640). The 38+P bullet ran about 970+ from a 3" M60--the same as the 357 round from a 2" barrel. The 38 Special version's bullet exits a 686-4" at about 1020 fps. I suspect the 357 round may not run over 1100. Does anyone have any chrono results of the GDSB 135-gr. round from a 6" or more barrel? I don't know of any.
On average, penetration of the 135-gr. GDSB bullets seems to run just under 11"; see Brassfetcher's tests.
I've not tested other 357 rounds--but I would be curious to know the velocity of a 125-gr. hot rod from a 2" barrel. I have shot some of those from a SP 101--and after a boxfull, my hand was bruised. I'll take the Speer 135-gr 38+p GDSB round any day over that in a lightweight. As ArmedBear notes, they are pleasant to shoot in a steel 2"--e.g., a 640.
The "FBI-type' rounds--in 38+P, a 158-gr. LSWC-HP running maybe 810 or so--shows a hair better penetration, about the same expansion--and a bit more / firmer recoil, but quite shootable as well. Now, if Remington / or whoever would do a 357S12--i.e., a loaded-down 357 with that 158LSWC-HP running at 900 fps from a 2" barrel, they would probably have a real marketable round to go against Speer. That's about the maximum reload I can shoot from the M&P340 and still reload, and meet Old Fuff's "Quad Five" criteria.
Take all the factors together, and you have the makings of a carry package that is lightweight, tolerable to shoot, with effective penetration and expansion.
Jim H.