Why doesn't S&W use the same slide for .40 and 9?

[jedi391]

A co-worker has been running an M&P .40 with a conversion Storm Lake 9mm Barrel for over a year now and has not had any reliability problems and the gun shoots very accurately. I played around with the 9mm in the .40 slide and found that the extractor fully engages the case and ejection is positive. Given that .40's don't have accuracy issues and from what I've experienced guns using the Storm Lake barrel are both accurate (I'm talking accurate) and reliable and some knowledgeable people believe the 9mm slide itself is a big part of the problem.....why not just use the same slide with some breach face changes?

[concreteguy]

Many believe the accuracy issue with M&P 9 has more to do with the rifling rate.

[Hwikek]

The cause of M&P accuracy issues has been debated for long enough that most people's answers have been to buy a VP9, PPQ, FNS 9, or Glock. What is causing the accuracy problems seems to be a combination of S&W not really caring, a lack of people shooting beyond 7 yards, and a number of different mechanical issues that when solved individually do little to fix the problem. I think C4i has more info about the problems that are supposed to be fixed with current M&P 9s.

[Clint]

The channel in the slide is the bolt face that guides the rim under the extractor.

A proper design will size the width of this channel appropriately for the cartridge used.

So, the channel and corresponding barrel lug is wider for the 40 vs the 9.

The 9mm conversion barrel has a ".40 width" barrel lug to fit the slide properly.

You can expect a 9/40 conversion to have lower reliability than a dedicated 9 barrel and slide.

[jedi391]

I get that. That's why I'm wondering why they just don't size the channel/bolt face for a 9mm and leave the rest the same as the .40 slide. Betting the PVD finish is one of their attempts to fix the slide problem but there's been no real indication one way or another that the new LE model has fixed the accuracy. This seems like a relatively easy fix.

[MegademiC]

I wondered this as well as the 40 have good accuracy and I havnt seen any complains for conversions. Seems sw should have done this after the first reports, imo. Fwiw, I have a 40 that's very accurate and a 9mm conversion bbl that is accurate, havnt gotten a chance to really see what it can do yet, but i was getting good groups so far.

[jayfl]

I get that. That's why I'm wondering why they just don't size the channel/bolt face for a 9mm and leave the rest the same as the .40 slide. Betting the PVD finish is one of their attempts to fix the slide problem but there's been no real indication one way or another that the new LE model has fixed the accuracy. This seems like a relatively easy fix.

I don't know if it's necessary to do so. The newer barrels seem to have a much tighter slide/barrel fit on average and the accuracy seems to have increased as well.

Anecdotal but just dropping the barrel from a 2014 production M&P9 into my very inaccurate 2011 production gun sorted the accuracy issues I was having. I bought a newer model replacement barrel from G&R as a more permanent solution in 2014 and that one shot much better than the old factory barrel as well.