Ok, I have some time and I've been meaning to put something like this together so, buckle up...
Initially the S&W M&P semi-auto pistols (not to be confused with the M&P revolvers) came in 4 calibers:
- 9mm
- .40S&W
- .357SIG
- .45ACP
The 9mm, .40S&W and .357SIG shared the same frame while the .45ACP was slightly larger.
The .357SIG and .40S&W even shared the same magazine. Alas, the .357SIG was dropped from the line early on.
All of them came in two sizes initially:
9mm and .40S&W
Full size - 4.25" barrel
Compact - 3.5" barrel
.45ACP
Full size - 4.5" barrel
Compact - 4" barrel
The compact models had a shorter grip.
A little bit later S&W offered a 5" model of the 9mm and .40S&W as part of the Performance Center version with a slightly better trigger. They also made a C.O.R.E. edition that had a cut out in the slide for an optic and taller sights to co-witness with that optic.
To add confusion, S&W offered each caliber in a stupendous amount of configurations. With a thumb safety, without a thumb safety, with a mag safety, without a mag safety. Then they offered different colors, black and FDE. When you consider all the possible combinations of color, safety, C.O.R.E., frame size, caliber and Performance Center, you come up with a dizzying array of models.
This is why people are asking for pictures.
All the models have the caliber stamped on the top of the chamber like this:
And on the slide like this:
The compact models have a "C" after the caliber on the slide like you can see on this:
When they came out with the 2.0 series, they added M2.0 on the slide. Like you can see here:
There are other differences, but I've covered enough of the major ones to tell basically what model you have. Even so, there is a way to tell exactly which model you have and that is with the SKU number. S&W uses this number to not only tell what caliber and size your gun is, but what configuration and color it is. If you have the original box, it will have a sticker on it like this:
You can take the SKU number and enter it in the search field on this site:
Smith & Wesson | Smith & Wesson and it will tell you exactly what model you have and all the features. The SKU is not on the gun anywhere.
I hope this helps a little.