I recently measured a newer S&W model 64 and found that a .357”+ pin gauge would pass and a .3575”+ gauge was a no-go. This mirrors what I have found with many S&W 38/357 revolvers. The S&W family appears to be very consistent in this regard. To obtain accurate measurements, the throats must be perfectly clean. This may require removing the cylinder from the gun and spending some time with an appropriately sized bore brush, a Lewis Lead Remover, or some other tool. A short, non-rotating cleaning rod is best for this task.
Slugging the bores of various S&W revolvers reveals groove diameters are also around .357” and consistent (although I have encountered some thread choke on some examples but not nearly to the extent that some Ruger’s exhibit). Colt is an entirely different ball game and I seldom play in that sandbox.
I previously sized bullets to .358” for use in S&W and Ruger revolvers but abandoned that method and I now size to .357”. That change to .357” increased accuracy slightly and decreased leading. It re-affirmed all the old instructions to size to the cylinder throat.
The cylinder throat is the last sizing die the bullet will pass through before reaching the barrel and it has the final say. It is my belief that a snug but not excessively tight fit is what is needed. A bullet that will pass through the cylinder throat with just a little bit of force applied is ideal.