Groups at 50 yards firing factory .38 Special wadcutter ammunition in a .357-chambered revolver will be about 15-20% larger than firing the same ammo in a similar revolver with .38 Special chamber. I tested this at Ruger comparing ten-boards of revolvers, refitting them with .38 Special cylinders and shooting them again. We also tested the .357 cylinders with SAAMI chamber and again after cleaning up with a "Bob Day .357 PPC" reamer which reduced the transition angle. A useful mod.
In my experience the most accurate "production" .357 revolvers were the original Colt 357, the Trooper .357 and the Python, due to their 14" twist barrels. The Ruger Blackhawk, properly tweaked and fed will do very nearly as well.
I never got as good grouping with the S&W Models 19, 27 or 28, despite access to Ransom and Broadway machine rests and a 50-yard test tunnel. But I learned that you could make the Ruger "Six" series really drive nails simply by fitting a Python barrel onto them and reducing the chamber to cylinder throat transition angle!
So, if you want a .357 revolver which really shoots well with .38 Special wadcutter with grouping, approaching the industrial test barrel, you must to reduce the transition angle from case mouth diameter to cylinder throats from 15 degrees, Basic (the half angle per side) to 6 degrees, Basic.
Accuracy is also noticeably improved by using the 1:14" twist of rifling of the Colt, as opposed to the 18-3/4" twist of the S&W. An even faster twist, as much as 1:10" was favored by a few gunsmiths building PPC match guns, Bill Davis, Bob Day and Jim Clark to name a few of the known notables.
Numerous gunsmiths, such as Reeves Jungkind and Bob Collins fitted Colt Python barrels to S&W or Ruger frames, aka Smolt or Cougar, which work very well. The faster twist is key!
Bill Duncan of Coffman Cove, AK has published several articles in The Fouling Shot issues #258 and #271, in which he used a scoped Bob Day PPC gun with 10" twist 9mm spec barrel to maintain linear dispersion in proportion to range, producing shot groups under 4 inches at 100 yards and shooting amazing groups with plunging fire to 200 yards and the HBWC bullets remained stable.