I have one of the older M&P 40s serial # MPM4xxx. I have never had any problems with the gun until today. I have put roughly 500-600 rounds through it without a hiccup. I noticed today that on the last round one of the magazines would start ejecting out on its own. Needless the say, the slide would not lock open either when this occurred.

At first, I thought that it may have been a magazine problem since the other magazine did not have the same problem. However, I bought a replacement mag immediately at the same range session. The brand new mag was getting ejected even midway through the magazine.

I tried shooting the gun one handed just to make sure it wasn't my grip. The gun was spitting the mag out. I let other people shoot the gun with similar results. Needless to say, the gun is going back to Smith-Wesson for a checkup. I suspect there's something wrong with the mag release. Has anyone else had this issue before?

Early production M&P 40 made back before late '07?

S&W identified an improperly heat treated steel insert in some early magazine catches. The inserts apparently weren't hardened right and could become worn and start to release the mag unexpectedly during recoil.

S&W was sending out new mag catches to anyone who called and reported that they were experiencing such an issue in one of the early production guns. The owner's manual contains instructions on how to replace (and reverse) the mag catch on page 26 of the current manual (which can be downloaded http://www.smith-wesson.com/wcsstore...tol_Manual.pdf ).

Otherwise, S&W would probably be glad to examine your M&P and make the necessary repair for you under their owner's warranty.

Making sure it isn't your grip causing the condition is a reasonable precaution, but there were some small number of mag catches with the improperly hardened steel inserts in them used in some of the early guns, too.

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Ret LE Firearms inst & armorer