DIY MPX Mag Modification: +5 rounds

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GunMag Warehouse shows a simple method to add capacity in your Lancer MPX magazines.

The MPX magazine has a floor plate with a protrusion. This protrusion hits the bottom of the follower.

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All you need to do is chop the protrusion down and this will allow the follower to move further down the magazine. Do it right you can get five more rounds in your MPX magazine increasing the capacity to 35 rounds.

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Now any modification is your responsibility. If you are not comfortable chopping parts of your $50 magazine, you can get the Springer Precision magazine extension. They retail for $35 or $30 each if you order more than five.  The website says “DO NOT USE THE BOTTOM PLATE ON YOUR MAGAZINE SPRING” so this seems more like a +5 magazine extension since there is no longer a protruding plate to prevent the follower from stopping early.

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Nicholas C

Co-Founder of KRISSTALK forums, an owner’s support group and all things KRISS Vector related. Nick found his passion through competitive shooting while living in NY. He participates in USPSA and 3Gun. He loves all things that shoots and flashlights. Really really bright flashlights.

Any questions please email him at nicholas.c@staff.thefirearmblog.com


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  • Travis Jennings

    Yes, getting 35 rounds is possible, but there is no extra room, which means inserting a mag of 35 on a closed bolt does not work. I chopped all mine, and I am plenty happy with 34 rounds.

    • Nicholas C

      I prefer to load on an open bolt. Shoot to slide lock and when making ready open and lock the bolt back.

  • Austin

    Looks like if the retainer went on the outside of the spring instead of in the middle it could hold even more

  • thedonn007

    So, why is it designed that way in the first place?

    • JumpIf NotZero

      Binding most likely. Over temperature, dirt, ammo tolerance, vibration, etc.

      Most people are dead wrong when they think about springs having a life from just compression, but there are some effective truths to spring rates.

      If they could have made the mag reliable at 30+ rounds, they would have.

      • Edeco

        I don’t disagree but another thing that comes to mind is tthat having extra spring length and not bottoming it out would help with longevity. No idea if thats what SIG had in mind.

      • thedonn007

        Yea, that was kind of my point.

      • Cal.Bar

        What makes you say that??? They could just have easily started with 35 round mags and the ATF, corp suits or other malifactor forced them to drop it to 30 and the cone was cheaper than retooling for the shorter mag body. Who knows.

        • JumpIf NotZero

          The ATF doesn’t give 1/2 a thought to magazine size on a domestic made gun.

  • Spencerhut

    The engineers that designed the MPX mag don’t know anything. Chop away* . . .

    *I own the company that makes the magazine springs for the MPX mags**

    **That is a joke, but seriously, the springs will be ruined.

  • Gary Kirk

    I’d be worried about over compressing and subsequently over lapping the spring coils.. 5 more rounds is nice, but not at the cost of reliability

  • It could have been designed that way for several different reasons. 1. More rounds = more drag on the follower so it might not have been able to keep up with the bolt and failed to feed sometimes or 2. more drag on the bolt resulting on less reliability on cycling when the mag is full or 3. the feed lips on the magazine might fatigue faster and crack under the higher load a more compressed magazine spring or 4. base plate on the magazine might have fatigue issues with higher spring loading or 5. the mag catch can be problematic with the increased spring load or 6. the magazine spring may be over stressed so far compressed.

    Of those listed I would say 4, 5 and 6 are least likely the original issue and 1,2 or 3 the most likely culprit.

    • You missed one:

      The lift velocity across the sweep of the follower in the unmodified magazine is much more consistent than it is with the full spring compression.

      This and the other reasons you mention are why one shouldn’t modify magazines in this way; the person who designed the magazine probably knows quite a bit more about magazine design than Bubba does.

  • Christian Hedegaard-Schou

    When used correctly, a spring won’t lose any qualities by being compressed for long periods of time.

    However, it’s entirely possible to over-compress a spring.

    I would put money on this mod substantially reducing the life of the magazine spring.

    Now, if replacement MPX springs are cheap and easily available, maybe it’s still worth it? But that ignores any functional problems that could also result from having too much spring pressure on the feed lips or bolt during use.

    I’d like to think engineers do things a certain way for a good reason. If they thought they could give us 35 rounds over 30, maybe they would have?

  • Gorilla Biscuit

    So how much is the replacement part, if this fails to function later on down the road? Is this a replacement part???????????????

  • Mark

    My opinion is that the same mag body will be used for .40 and .357 sig and still have 30 round capacity.