New M&P 40c, first outing 200 rounds, but not trouble free. This my first striker fired pistol and first .40 cal. Love the feel and the accuracy. Problem was random slide locking back during firing. No stovepipes or jams. The slide would be locked back and just a slight tug on the slide would chamber the next round. Both mags and 3 brands of ammo and with three different people.

Any ideas??

3 brands of new ammo?
Did you field strip and clean/lube it prior to initial shooting?

Be sure one of your thumbs is not riding the slide stop lever. Shoot the pistol one handed making sure your thumb is not contacting the lever and see if it still randomly locks back with rounds still in the magazine. Bill

I was thinking the same thing...but three different shooters swayed me on that.

Cleaned and lightly lubed prior to and again after first 100 rounds.

May have the answer with the thumb position. Current thumbs forward style I am trying to get used to puts my strong hand thumb just under the slide release. Was no issue with my 9MM and my friends 9mm's by Ruger , SIG and Glock. All three of us were using a thumbs forward grip and were shooting the snappier .40 for the first time

Will update after testing the theory next weekend. Thanks for the suggestions

Update!

Hate to say it but my new M&P 40c is going back to S&W.
Put another 100 rounds through it this morning after another thorough clean and lube. It is getting worse. Thumb positively clear of slide release, two brands of ammo and two shooters. Minimum 1 slide lock per mag and a high of 4. I forgot to mention last time that the slide release is impossible to operate without two thumbs and all my strength. Operates freely when pressure of slide released. I always rack a slide to release it so was willing to overlook that. Nothing obvious to my eye but slide tab must be out of position.

Totally sucks, but it happens. This will be my first ever return to any manufacturer in 50 years of shooting so I have been spoiled.

A couple questions if I may...

When you say, "...the slide release is impossible to operate without two thumbs and all my strength" is there an empty mag in the gun?

With the slide removed, does the slide stop want to stay down? It should have a spring pulling it down. If there is no tension from the spring, or the spring is missing, I could see where it will accidentally hold the slide back sometimes.

Look at this pic:

I took this from the Parts Identification post: M&P Parts Identification
However, I added a blue arrow pointing to the slide stop spring. Look inside your gun and see if this spring is there. If it isn't, that could be your problem.

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Freedom isn't free.

Slide stop stays down with the slide removed. Will let S&W figure it out and FIX IT! I traded a carry gun to get this one so pretty bumed now. We shall see how S&W handles this. My friend got a Ruger SR9c the same day and the Ruger has been flawless. Am currenrly eating crow big time!

Sorry to hear it's going back. I have a 40c and a 357c that both went back due to a similar problem...slide would lock with rounds in the mag...both of them! Sent them back together and Smith turned them around in 3 weeks. The both perform 100% now and I'm quite happy. A bit of a drag to send them in but Smith took quick care of the issue so not much to complain about. You'll like it when it returns...

As for the SR9c...my son in law has one and while it is a good gun his went back to Ruger for a fix...happens to all of 'em at one time or another.

UPDATE! Happy Happy! Almost a month from first e-mail to S&W till gun back in my hand. 21 days from shipping till return.

200 rounds with four brands of ammo, no problems. Hand sore from that much 40 cal. but loving it

Now it's at the top of the carry list

What did S&W do to it?

Replaced slide release and mag release.

great news!!!!! hopefully you wont have any problems from here on out.

Glad to hear all is well.
I'm not a fan of pressing the slide lock lever to release it.
I've been trained to use the slingshot let it ride method...

Same here, I just think you should be able to use it without both hands. Much better now, can release one handed with hard effort after testing session, could not when took it out of the box. Every semi auto i have ever owned I could use the slide release with a easy press of the thumb, why should the M&P take a deliberate hard determined effort after several hundred rounds of break-in?

My 40c has done that 4 or 5 times over the 400 or so rounds I have put threw it so far, but I have found that when I tighten my grip up it doesn't do it. I have put it down to limp wristing it.

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Steven M. Howard USAF Retired

I have the same issue when using my small backstrap. Problem no longer present with the large or medium backstrap. Just a fraction of an inch smaller and my thumb inadvertently rides the slide stop. Food for thought.

Did 200 rounds with thumb absolutly away from the slide stop, one to four locks per magazine. after S&W repair had one lock in 200 rounds and that was with cheap Russian ammo, no problems with other 3 brands of ammo. I call it fixed.

Gave the bitch the grip from hell, no help. Not the problem

Ya, from further reading on some old posts the fix always seems to be the slide stop like they did to yours, I just got my CORE 9 back from them a month ago so not really wanting to send this one in, so I ordered a new slide stop from GandR Tactical. And after thinking about it I could see my grip causing the round to jam the slide, but mine is locking back with the round flush in the mag, and it has done it with all 3 mags, but only five or 6 times total.

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Steven M. Howard USAF Retired

Well I received my new slide stop from GandR Tactical and installed it, went to the range today loaded all 3 mags up and fired 50 rounds total with no random slide locking open malfunctions. Will have to keep an eye on it for a while but I hope this fixed it. The spring on the old one was loose compared to the new one at the point where it mounts to the slide lock.

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Steven M. Howard USAF Retired