I usually try to stay positive, but geezzzz, please understand that there are experienced shooters posting about the accuracy issues of the full size M&P 9mm. This is real.
Congrats on having 4 guns that shot consistently. Some of us aren't experiencing that. I received a new barrel for mine, purchased in May, and went from unable to group, unable to keep 5 shots on the paper at 25 yds (benched and bagged), to geting 4-5 inch groups at 25 yds. I still get the occasional unexplained flyer. And anyone who wants to get pissy about shooting out to 25 yds, well, I'm not going to try to explain it to you, you don't want to hear it. (For some shooters, that is the Gold Standard of measured accuracy. That doesn't mean we are practicing to shoot defensively at 25 yds, unless required during a drill or shooting station).


As far as proven design.... It is my understanding that there were changes made to the barrel production when the compact was introduced. So if that is true, the proven design was altered.

As far as QC, my son is an engineer for Micro VU, which makes optical scanning equipment for QC inspection of parts put into production. Used in many, many industries. This is what S&W needs, to pull parts out of a production run to check if they meet the specs of the "Proven Design" benchmark. There are a lot of variables that can change during a production run of a part.

What I can tell you is that the replacement barrel that I received has a much tighter fit to the slide. Hard to really tell how it is interacting with the locking block.
Bottom line, it shoots better.... dramatically better. I don't think I changed very much while I waited for the gun to return from S&W. Infact, I did not fire a single round in any handgun while I awaited its return.

Cw