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Update on converting 15 round M&P 9 magazines to 17 round capacity

I told you guys a bit back that I cut the extra spring to stuff two more rounds in each. I’ve now ran each magazine a total of four times. And this last time, I had my first failure using one of the WECSOG inspired magazines. On the second round, there was a failure to eject. Old me on the range would have stopped shooting, started looking at it, and trying to figure out what exactly went wrong there. But new me on the range just tapped and rack. The rest of the rounds ran fine. It was steel case TulAmmo.

4 Responses to “Update on converting 15 round M&P 9 magazines to 17 round capacity”

  1. Paul Koning Says:

    There’s a neat way to make a spring that doesn’t take up as much room. Boberg used it in the XR45, though it couldn’t be patented because it had been done many years before.

  2. wizardpc Says:

    My m&p9 does that about every 4th round with tulammo. No mods required!

  3. mikee Says:

    Paul’s comment made me go look. Not only was the Boberg baseplate held on the magazine by a flat, tight-wrapped spring end, rather than making it larger and using the magazine body to hold it, the spring itself has decreasing diameters on its helical loops as it progresses up the magazine. There is no follower. Elegant solutions to save a few tenths of ounces and millimeters.
    https://assets.americanrifleman.org/media/2281489/b_boberg_dis1s.jpg?width=526px&height=451px

  4. Paul Koning Says:

    Yup, it’s just enough to fit in one more round. As Arne put it, with a standard spring the magazine would hold 5.5 rounds; now it holds 6.

    The patent I mentioned is 4,397,453, from 1983, assigned to Seecamp. The drawings are very good.

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