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Thread: M&P40 1.0: Slide Won't Lock Back After Last Round with Some Magazines

  1. #1
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    M&P40 1.0: Slide Won't Lock Back After Last Round with Some Magazines

    I've got a relatively early full size M&P40 with 12 magazines bought at varying periods. I recently sent it in to have the slide stop replaced due to the right side tab having broken off at some point. Since I got it back from S&W, the slide won't lock back either after the last round or when I manually retract it with some of the magazines. I didn't notice this problem before I sent it in.

    Of the 12 magazines, eight have a polished finish and four have a matte one. The matte ones work fine; four of the polished ones lock the slide back but the slide stop doesn't seat fully in the notch in the slide, and the other four just don't lock the slide back at all.

    It looks to me like there could be three reasons for this issue - a) some of the mag springs are weaker due to age or design; b) the follower design was changed at some point; or c) the spring on the new slide stop is stiffer than the old one. I compared the slide stop tension on the 40 to the one on my M&P45 1.0 and it does seem stiffer, but the specs for each gun could well be different.

    My guess at this point is that the problem is with the magazine spings. I think that the springs in the ones that don't work don't apply enough upward pressure on the follower to push the tab on the slide stop up to engage the slot in the slide. But since I don't know my ass from a hole in the ground about this stuff, I'd appreciate any input from knowledgeable folks.

    Wolff has ten packs of their +10% M&P mag springs, if I need to go that route.
    Last edited by revchuck38; 12-01-2017 at 10:07 PM.

  2. #2
    The M&P slide stops are stamped steel and it's possible that the portion that engages the slide has a burr or a little too long. Since you didn't have the problem before I'm thinking the slide stop is the issue.

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  3. #3
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    Join Date
    Sep 2017
    Location
    South Louisiana
    Mystery solved (I hope). I have nine M&P9 mags left from when I had an M&P9. I tried them in the M&P40 and they all locked the slide back properly. I disassembled one of the 9mm mags and one of the malfunctioning 40 mags and the spring from the 9mm mag was a good inch or so longer. I assembled the spring from the 9mm mag and the 40 follower and put them in the 40 mag tube and tried it - perfect.

    Now I just have to order the springs.

    ETA: Midway had the factory springs in stock - $21 for eight of them, including shipping. Cheap fix!
    Last edited by revchuck38; 12-02-2017 at 09:03 PM.

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