Much to my surprise, every one of the factory rounds I checked easily dropped into every chamber of all the .38/.357 guns.
A relic Model 37 with alloy cylinder that I put back in firing condition would accept the .38 S&W in two chambers but not in the other three.
All chambers had been cleaned throughly.
The two chambers that accepted the .38 S&W appeared to have bulged a bit, which was a not uncommon problem with the early alloy cylinders. They had recalled the alloy cylinder guns and replaced with a steel cylinder, which makes those with the alloy cylinder a bit more collectable.
If you have a .38 special or .357 magnum that will accept the .38 S&W you either have undersized .38 S&W cases or swollen or otherwise slightly oversized chambers.
.38 S&W neck dimension is .3855 with base .3865.
.38 Special neck is .379, base is .379.