Given the ctgs of the day it's hard to imagine S&W revolvers having catastrophic failures.
There is no indication I am aware of to suggest there had been any Cylinder failures in the m1899 or subsequent Models of the M&P or K-Frame Revolvers prior to the early 1920s.
And, by sometime in mid 1899, the .38 Special Cartridge went on to being a Smokeless one, equal to or surpassing the SAAMI Loadings of to-day, so...it is not as if the initial Cartridges for the .38 Special Revolvers were at all any less powerful than those of our present time. And, many people believe they were in fact more powerful than those of to-day.
Something had to prompt them to add a manufacturing step. It's not like S&W was using junk steel or cast cylinders like the cheap imports. Given the numbers of non-heat treated guns that are still firing factory ctgs successfully after almost 100 years, it's hard to believe it was just the switch to smokeless powder.
Anyone know?
The 'switch' to Smokelss occurred more or less as the M1899 was being introduced. Cartridge Manufacturers simply made and offered Smokeless and Black Powder Cartridges then, since the Revolvers were good to go with either one.
While the .38 Special Cartridge was originally designed ( in 1898 I think ) to be a Black Powder Cartridge, the change occurred very early ( 1899 ).
As far as I gather, one could none-the-less buy either Black Powder or Smokeless Cartridges in .38 Special, at least till sometime into the mid 1920s.
Original Black Powder performance was listed as being 156 Grain RN Lead Bullet, and, 950 FPS ( but I do not know from what Barrel Length ).
If memory serve, this was with 21.5 Grains of .3F Black Powder, and, 'Folded Head' or 'Baloon Head' Cartridge Cases.
Improvements in Cartridge Cases prevent us from getting more than m-a-y-b-e, about 18 or so Grains of 3F Black Powder into a now-a-days .38 Special Cartridge case, unless we hold the Bullet out farther than usual, and, there is not much for 'farther' to hold it out, without the Bullet protruding from the Cylinder and interfering with the Cylinder's rotation, so...far as that goes...
I myself simply do not understand what is meant in this context by 'Heat Treating' and I do not know what was being done or what Steels or what Alloy was being used prior to so called 'Heat Treating' being introduced...nor once it was introduced.
Again - Alloys capable of standing .357 Pressures, in a K Frame, were long since available anyway, and could have been used in the m1899 right from the get go, and if they had wanted to, they could have introduced the .357 Magnum in 1899 and been fine...so, none of this makes any sense, or none of it makes any sense without the key information as for what Steels were being used, and, why they were being used ( machinability factors in relation to Tensile Strength, in relation to the particular Alloy's requirements for processes of Hardening, being Machined in an annealed condition , in semi hardened conditions, etc, avoidance of High Speed Steels oweing to Maching costs and troubles, distortions occurring if using certain Temperable Alloys if being machined 'soft' and then being 'Hardened', too costly to machine 'Hardened' Steels once Hardened to avoid distortions of post-machinging distortions, etc. Always there are a confluence of Reasons for the Alloy one uses or elects for a partilulat purpose or part, so, who knows? etc ).