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Thread: Make my own extended magazines?

  1. #1
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    Ok, long story short, I have a .40 caliber S&W SW99 that has 12 round magazines. Walther/S&W have made 20 round mags for the 9mm, but apparently have no plans of doing the same for the .40. Without a commercial source, I'm toying with the idea of essentially cutting the top off of one mag and gluing another to the top of it. Yes, I realize it'll be much more complicated than that, but you get what I mean. Do any of you that have experience with metal working or gun smithing or anything else that might be helpful have any idea if this could work?

    On the other hand, does anyone know of anyone who would do such custom work?

  2. #2
    Regular Member fullauto223cal's Avatar
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    Something else to check on would be the spring. Obviously one designed for a regular magazine would not work. If you could order a 9mm replacement spring and after having joined two modified magazine housings you could attach your .40cal follower to the top of the 9mm spring. Good luck in your fabrication efforts.

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    Yes....It CAN be done....just wouldn't be too reliable and would be awkward. For the gain in ammo....simply not worth it. Learn fast mag switching.

    It's been donein rifle mags that extend from the receiver quite often. Drums for the glock and 1911....try holstering that. Extended mags for other pistols have a ghetto cool (not knocking you ) but unless it's a FA, I wouldn't even try it. Just for playing, go for it.

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    TheMrMitch wrote:
    Yes....It CAN be done....just wouldn't be too reliable and would be awkward. For the gain in ammo....simply not worth it. Learn fast mag switching.

    It's been donein rifle mags that extend from the receiver quite often. Drums for the glock and 1911....try holstering that. Extended mags for other pistols have a ghetto cool (not knocking you ) but unless it's a FA, I wouldn't even try it. Just for playing, go for it.
    OhI wouldn't be carrying the things...they'd be for IPSC lol.

  5. #5
    Regular Member Michigander's Avatar
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    Are the mags metal or plastic? I'm guessing plastic since you said glue, but just in case they are metal, and so everyone knows, a guy who goes by the name "bustoff" on the web does that with metal mags. He cuts them, welds them together, including the springs, and refinishes them. A google search could help you find him. I saw a M11 50 rounder he made, and it was extremely well done. I think his conversions are like 50 or 60 bucks for labor, plus shipping and mags. I don't know that he does anything but MAC conversions since that's all I've heard of, but it doesn't hurt to ask.
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  6. #6
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    They're metal. I just used the term glue since it easily communicated the idea. I'm about to google the guy you mentioned.

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    This might seem like a radical suggestion, but will your .40 S&W SW99 retain a 20 round 9mm magazine if you insert it?

    I never used a S&W SW99, but do have a collection of SIGs. I have several 20 round 9mm mags for my P-226 in that caliber. They fit just fine in my .40/.357 SIG P-229 and I have had no problem with the the very slightly larger rounds feeding. I can get 15 or 16 rounds on a 20 round SIG magazine versus the standard 12. The longer mags do, of course, extend a couple of inches beyond the grip. Does SIG approve or recommend? I doubt it and haven't asked, but I do note that some sellers of 20 round 9mm mags advertise that they work fine for .40/.357 SIG.

    No idea if this is applicable to your .40 S&W SW99, but thought it worthwhile to pass along.



  8. #8
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    markand wrote:
    This might seem like a radical suggestion, but will your .40 S&W SW99 retain a 20 round 9mm magazine if you insert it?

    I never used a S&W SW99, but do have a collection of SIGs. I have several 20 round 9mm mags for my P-226 in that caliber. They fit just fine in my .40/.357 SIG P-229 and I have had no problem with the the very slightly larger rounds feeding. I can get 15 or 16 rounds on a 20 round SIG magazine versus the standard 12. The longer mags do, of course, extend a couple of inches beyond the grip. Does SIG approve or recommend? I doubt it and haven't asked, but I do note that some sellers of 20 round 9mm mags advertise that they work fine for .40/.357 SIG.

    No idea if this is applicable to your .40 S&W SW99, but thought it worthwhile to pass along.

    I'm pretty sure the 9mm magazine locks tightly into the frame, since the 9mm and .40 frames are almost identical. I know someone that has a 9mm P99, but unfortunately they're in a different state lol. I'll have to see if I can find a 9mm mag in the stores around here, but I doubt it.

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