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I have an M&P40 pistol. Yesterday I noticed that with an empty magazine inserted, and pulling the slide to the rear, that the slide did not lock back on the empty magazine. Upon further investigation it is doing that with all 4 of my M&P40 magazines. The spring in the M&P40 magazines is not strong enough to overcome the spring on the slide lock lever. I tried some magazines from my M&P9 and they lock the slide back properly. I will order some new springs for the M&P40 magazines.

I have been keeping 2 of the M&P40 magazines fully loaded for 6 months and letting the other 2 magazines rest for six months. I can't remember how many years that I have had this firearm. Is replacing the magazine springs from time to time just a reality, or is there a suggestion of a better plan or "best practices" to keep them from wearing out.
 

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Second the Wolff springs. Bought a bunch a few months back for my older M&P 9 magazines. Had similar issue after I put some +2 bases on some of them. New mags. worked fine, the old ones weren't locking the slide back. Pulled the springs and there was a big difference in length between the older mag. springs and the brand new ones. Two new springs with some old ones. When I saw that, I ordered a pack from Wolff springs. Cured the failure to lock back issues.

 

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Interesting. It seems that many problems can be caused by weak magazine springs. My S&W Shield EZ .380 would randomly stovepipe the last round of a magazine rather than allow it to feed into the barrel via the feed ramp: Not every time, but often enough to be irritating, and it happened with more than one of my 6 relatively new magazines. Someone on some forum suggested removing the springs and stretching them. That solved the problem - at least for now.

I agree that with older springs which have become compressed over time, the best thing is to buy new springs. They don't cost a lot.
 

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I have an M&P40 pistol. Yesterday I noticed that with an empty magazine inserted, and pulling the slide to the rear, that the slide did not lock back on the empty magazine. Upon further investigation it is doing that with all 4 of my M&P40 magazines. The spring in the M&P40 magazines is not strong enough to overcome the spring on the slide lock lever. I tried some magazines from my M&P9 and they lock the slide back properly. I will order some new springs for the M&P40 magazines.

I have been keeping 2 of the M&P40 magazines fully loaded for 6 months and letting the other 2 magazines rest for six months. I can't remember how many years that I have had this firearm. Is replacing the magazine springs from time to time just a reality, or is there a suggestion of a better plan or "best practices" to keep them from wearing out.
I've had my magazines loaded to capacity since April 2007, I went to the range 3 weeks ago all my magazines (6) functioned properly. You may want to check with S&W about the lifetime warranty if you are the original owner it can be verified if you registered it when purchased.
 
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