Howdy
There is a lot of misinformation about the 38 Smith and Wesson Cartridge on the Internet.
Smith and Wesson developed the 38 Smith and Wesson cartridge for their 38 Single Action,1st Model, Top Break revolver, which was manufactured from 1876 until 1877. This model was also known as the Baby Russian. So it follows, the cartridge has been around since 1876. This Baby Russian left the factory in 1877. Over the years, S&W made a great many more Top Break revolvers chambered for the 38 S&W cartridge, but this was the first.
Smith and Wesson developed the 38 Special Cartridge for the 38 Military and Police 1st Model (also known as the Model of 1899). This Model 1899 left the factory in 1899. The 38 Special cartridge was developed in 1898 as a replacement for the 38 Long Colt which proved to be under powered in the Philippine Insurrection. (Smith & Wesson always marks guns chambered for 38 Special as 38 S&W Special. It is the same cartridge, S&W just likes to mark their guns that way because they developed the cartridge.
Let's see what we're talking about. In the photo below, the cartridge on the left is a 38 S&W, the round on the right is a 38 Special.
Now let's look at the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) specifications for the two cartridges.
The case of the 38 S&W is ever so slightly tapered. From .3863 at the rear to .3855 at the case mouth. Bullet diameter is given as .361 +.000/-.006. So the narrowest bullet for 38 S&W would be .355. (When I load 38 S&W I use .359 diameter bullets, because that was what I was able to find.)
Case diameter for 38 Special is .379 without any taper. Bullet diameter for lead bullets is .359 +.000/-.003. So the narrowest lead bullet for 38 Special would be .356. Jacketed bullets are .358 +.000/-.003, so the narrowest jacketed bullet for 38 Special would be .355.
Here is my Colt Army Special, which I posted a photo of in post #27, loaded with 38 Special ammunition. These are my handloads, that explains the different headstamps.
In the interest of science, I attempted to load my Army Special with 38 S&W ammunition from 5 different manufacturers. A sample of the different cartridges has been laid out below the revolver. None of these rounds would chamber completely. Owing to the slight taper of the cartridge, and typical manufacturing tolerances, some of the rounds would enter the chambers part way, some would not enter at all.
However, if a 38 Special revolver has slightly oversized chambers, 38 S&W ammunition will chamber all the way.
One other thing to bear in mind. During World War Two, Smith and Wesson made over 110,000 VIctory Model revolvers for England and their allies. These revolvers were chambered for the 38 S&W cartridge, not 38 Special, because that round was more easily available in England. Many, not all, of these revolvers were rechambered to 38 Special when they were imported back into the US. This was done by cutting the chambers deeper for the 38 Special cartridge, but the rearmost section of the chamber was still the larger diameter for the 38 S&W cartridge. It is reported that because the rear of the chamber was slightly oversized for 38 Special, cases often split.