Slide fails to lock open on empty mag

[Dmartin]

I have had a sporadic problem with the slide on M&P 9 Pro (5 inch barrel) not locking in the open position at the end of an empty magazine. I’ve marked the magazines, and 2 of them seem to have the problem more frequently than the other 6 magazines I normally shoot.

The slide fails to stay in the locked open position with an empty mag about 20% of the time. There are 2 different styles for the plastic piece at the top of the mag spring, and 1 mag of each style of cap piece has been a problem. All are S&W brand and were purchased new. When I put an empty mag in and manually pull the slide back, the slide locks open every time, even with the suspect mags.

Since the problem I sporadic, I wonder if my right thumb might be contacting the slide release and unlocking the slide on the last round. If the cap piece was worn, I think the slide would fail to catch on every occasion. The picture is not great, but I think you can see the 2 styles of mag cap piece.

Has anyone else had this problem or found a solution?

 

[Houdini1953]

Make sure that the insides of those mags are clean after breaking them down. The magazine follower when in the proper position will push up the slide stop . The problem is that if you have a weak spring then you will continue to have the problem.

I know that a lot of people complain about how hard a new magazine is to load rounds into but really this is a good thing and it means it is working properly although a pain to the user!

Springs are cheap so if that ends up being the issue then it is no big deal!

 

[walkin' trails]

When that starts to happen, its usually the spring, but try to replace the follower anyway. Its polymer and wears against steel. Then again, it could also be your thumb as you said. That happened to me once. I replaced the springs and followers, and the problem persisted. I replaced the slide stop lever and still continued. Thinking I'd done some major damage to the pistol ( I landed in it when I fell on ice - hurt too - had "Glock" imprint on my skin) I packed it up and sent it to an armorere. The armorer did everything I did and replaced the recoil spring too. Got it back, still had problems, and finally caught myself thumbing the slide stop.

 

[Dmartin]

Thanks for the help

I'll try to keep my hand a little lower on the grip and see if the problem goes away-