Get ready I am about to ramble with pictures! I went and tried again just for fun. This time I tried three types of 38 S&W ammo.
Left to Right: PPU 38 S&W, Remington 38 S&W, and my own reloaded 38/200 in CBC (Magtech) 38 S&W brass (got to feed the Webley good and proper ammo).
My S&W 442 Moonclip (cause moonclips RULE!)
Remington drops right in, PPU almost goes, and my 38/200 goes almost half way.
S&W Model 10, Remington goes all the way in with very gentle pressure, PPU, and 38/200 goes about 1/4 of the way.
S&W 627, Remington about half way. PPU and 38/200 don't get past my heavy chamfer. (yes she is dirty, I like her that way)
Ruger Blackhawk Convertible, Remington drops in, PPU almost goes almost all the way, 38/200 just starts.
I took some measurements of just a couple of each and mentally averaged them. They were pretty consistent in their own lots.
Remington
Mouth: .3800
Base: .3820
Bullet: .3565 (Though technically in SAAMI spec, Remington you lazy bums use the right bullet diameter!)
PPU
Mouth: .3825
Base: .3835
Bullet: .3610
My 38/200 reloads (CBC brass)
Mouth: .3835
Base: .3835
Bullet: .3610
Now if we look at SAAMI spec for 38 S&W ammunition
Mouth: .3855 +.000/-.006
Base: .3863 +.000/-.006
Bullet: .3610 +.000/-.006
If we compare that to permissible 38 Special chamber
Mouth: .3801 +.004/-.000
Base: .3809 +.004/-.000
Given the evidence above and an analysis of the SAAMI specs it is completely plausible to have certain in-spec 38 S&W ammo that will go into in-spec 38 Special chamber and yet in another incident have in-spec 38 S&W that will not go to in-spec 38 special chambers. Tolerances can be a bitch...
Rambling...