Introduction
I posted this yesterday (on page 26 of a different thread and in slightly different format) whose topics wandered as they tend to do. In order to see if this has any merit I'm posting it now separately. I'm proposing a model that explains some - but not all - of the well-know S&W accuracy problems in order to provide a conceptual framework for those (including me) who have struggled to understand the "why" of these problems. As we say in experimentation, even if it does not prove to be valid, we've at least provided negative data points... If it's valid, then we may have a way forward.
Issue
Smith & Wesson M&P9 pistols (9mm) are pleasant to shoot and have good ergonomics but suffer from accuracy problems that show up at 25 yards but are not always apparent at shorter ranges and are generally not also present in their .40 and .45 caliber models.
Hypothesis
This hypothesis holds that in order to keep recoil forces and slide velocity under control in their lighter-recoiling M&P9, more material is removed in production from the slide than from those of .40 or .45 caliber pistols. This lower thickness is then more subject to distortion when undergoing heat treatment than the thicker slides (this is the heart of this hypothesis). These resulting warped slides observed by others are then the cause of some of the reported inaccuracies. Warped slides explain why barrel replacement is not a fix for all of these pistols with accuracy problems - this has been observed by others. (If it is not true that M&P9 slides are thinner in places than those of .40 or .45 calibers, then this hypothesis fails.)
Possible Verification Test
The hypothesis should be easy enough to test. As I understand it (not having an M&P), you can fit a 9mm barrel in a .40 cal slide. If it performs poorly in the 9mm slide but properly in the 40 slide, then we're on our way to an answer, if not a solution ( a "solution" would be up to S&W). I do recall, however, that Bill Riehl discovered some suspect barrels (chamber concentricity, etc.), so I wouldn't expect those to work well in any case; a barrel may mis-perform in both slides.
Disclaimer
Data on warped slides and non-concentric barrels certainly came from Bill Riehl's testing. Other observations likely came form several combined sources. I'm neither an engineer, nor a gunsmith, so I'd be interested in hearing the no doubt infinite ways in which this is wrong... Please let me know what you think.
(Okay, the format's a bit overwrought, but I've been retired for awhile now, and who could foretell that I'd miss formal written reports?)