I own the M&P version of this scandium J-frame .357, and I've had
none of these problems:
This model has a stainless steel cylinder, coated with what appears to be melonite. The worse that has happened is a little bit of what I would call "scorching" on the front face of the cylinder, which is apparent in this picture. I have fired 110 grain .357 Winchester White Box in this gun, as well as my 125 grain .357 Critical Defense carry load, and even a 158 grain jacketed soft point white tail hunting load.....which beat the hell out of my hand, but didn't seem to hurt the gun one bit.
I initially bought the M&P340 when I was shopping for a PD model, but the store I bought from didn't have the PD. I "settled" for the M&P. The reason I initially thought I had "settled," was because I really had my heart set on the additional 2 oz less of weight that the PD offers compared to the M&P. But in hindsight, 2 oz doesn't matter a hill of beans when you're comparing an 11 oz gun to a 13 oz gun—both are insanely light—and I'm actually happier for having the more robust cylinder of the two pistols.
I also prefer the huge Trijicon "express" front sight on the gun. This is not my primary carry weapon, and all of my carry guns have Trijicon sights on them, but with the exception of the orange Trijicon HD sights on one of my semiautos, this system on the M&P340 works better than the rest of of them.
The M&P340 is the best snubby I've ever owned, and that includes an 642 I used to own, and a 2" 640, plus the 642CT that my wife still owns (and never carries because she can't handle the trigger pull....something about carpel tunnel...).