JimmyR said:
I/M- itty bitty frames, not in production anymore, most chambered in 32 S&W Long or similar.
J- Examples include the Model 36, 642, etc. Smallest S&W in current production, commonly chambered in 38 special, with some chambered in 357.
This really isn't quite comprehensive, so I feel obligated to fill in the blanks, since I tend to do that.
The
M frame was only used for the .22 Hand Ejector aka Ladysmith, a truly diminutive revolver that is closer in size to today's NAA mini-revolvers than any currently produced Smith. The .22HE was offered only in a .22 Long (NOT Long Rifle!) 7-shot configuration and was only produced in the early 20th century.
The
I frame is comparable in size to the current J frame. It exists in 3 major variations.
The first I frame variation was used solely for the .32 Hand Ejector 1st Model, produced in .32 S&W Long only from 1896 to 1902. This type has no front ejector rod alignment lug, the cylinder is opened by pulling the ejector rod forward, and the cylinder stop is located at the rear of the topstrap; the latter two features are unique to this model only. A leaf mainspring was used.
The second I frame variation added the familiar front ejector rod alignment lug and push-forward cylinder release latch, the cylinder stop was moved above the trigger guard, and most frame and cylinder dimensions were changed. The leaf mainspring was retained. This version was produced from 1903 to 1952 and was used for compact revolvers chambered in .22LR, .32 Long, and .38 S&W (but NOT .38 Special; that was the J frame).
The third I frame is widely known as the "Improved I" frame. This version was slightly enlarged in several dimensions, notably by lengthening the grip by 1/4" and enlarging the trigger guard. The leaf mainspring was replaced with a coil mainspring. The Improved I was produced from 1953 until 1960-1962, during which time all Improved I frame models were gradually replaced with an equivalent J frame model.
The
J frame was originally introduced because the I frame's cylinder window was too short end-to-end to fit a .38 Special cylinder. The J frame also exists in 3 major variations.
The early version used from 1950 to 1952 had a smaller trigger guard and 1/4"-shorter grip frame- the same dimensions used for the original I frame, although the J frame has always used a coil mainspring, whereas the early I frame used a leaf mainspring like the larger frames did (and still do). The early J frame was only used for the spur-hammer Chief's Special (aka pre-Model 36), and collectors refer to this variant as the "Baby Chief" or "short-grip Chief".
In 1953, S&W enlarged the trigger guard and lengthened the grip frame, to correspond with similar dimensional changes that created the Improved I frame. This version of the J frame was subsequently used for the concealed-hammer "Centennial" and shrouded-hammer "Bodyguard" (NOT to be confused with the current polymer-frame all-caps "BODYGUARD 38") variants, along with the aforementioned I frame replacements.
In 1996, S&W introduced the J Magnum frame, which was lengthened yet again to allow a .357 Magnum cylinder to fit. S&W simultaneously introduced (the dreaded) MIM lockwork with a flat-faced hammer and full-floating firing pin on centerfire models. (This style of hammer and firing pin was actually used on all rimfire Smiths produced from mid-1935 onward, but the previous mechanism was slightly different.) The frame size was changed simultaneously across the entire J frame line.
Every I/J chambering has been offered in the larger K frame at some point, which causes confusion among S&W n00bs, although the I/J .38 S&W and .38 Special guns can be easily differentiated because the compact frame dimensions limit capacity to 5 shots, whereas all .38-caliber K's are 6-shot. (FWIW a 7-shot .32 S&W Long K frame was designed but never produced, and later .22LR K's are 10-shot; likewise, recent J frames are 8-shot in .22LR and 7-shot in .22WMR.)
The current polymer-frame
BODYGUARD 38 (spelled in all caps in S&W literature) is similar in size to the J frame, but is substantially different in numerous respects, notably the absence of a front ejector rod alignment lug and the use of a polymer frame, a unique top-mounted cylinder release, and a totally new cylinder advance and lockup mechanism that does away with the external cylinder stop altogether. S&W has sometimes lumped this model together with the J frames in their catalog, presumably due to its size, but it's really a totally different design.