EXERCISINGTHE2ND, None of my M&Ps have the internal lock, so at least in my case they are a non-issue. I have not heard of any firing-related problems traced to the internal locks in the M&P series (unlike the revolver system).

I think the issue is more related to the "one size fits all" system in use for various parts of the M&P platform. Except for barrels and mags, pretty much every other part will interchange between the 9mm, .40 and .357 Sig, caliber pistols. Thats great for mass production; less so for tuning a pistol to different calibers of differing power levels. For example, the same strength recoil spring assembly is used for the above three calibers. Works, yes but probably not optimal.

Midway had slide stops in stock and I needed some other stuff anyway so I've got one on order. There are a few tweaks I want to try to confirm that the problem is actually related to the slide stop. I figured that if I screw it up, I still have the original one to reinstall.