MP5 40 cal receiver cracking
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Thread: MP5 40 cal receiver cracking

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    Default MP5 40 cal receiver cracking

    Have a fairly new German HK MP5 40 S&W with a receiver that's starting to get hairline cracks.

    Gradually more as more rounds are put through. Using .40 reloads with the lower charge

    Cycles fine, about 1000 rounds through it.

    It has the LO bolt and I'm certain my ammo is low powered.

    I don't know enough about the .40 but haven't found other references online about the receiver cracking (both sides along the side walls between the scope mount and rear sight block area

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    Post some pics if you can sounds serious
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    Is this a post sample gun? Or was it built using a German kit?

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    VX666

    I am going to guess that it was made from and HK94 and a 40 parts set. If it such a weapon, could the cracks be along the weld piecing to two
    together?

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    This /\/\/\

    Only a reweld/graft would develop cracks of this nature since it was most likely done to retain some markings from the original donor.

    Pics would help of both the inside outside of the receiver
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    Special weapons receiver ?

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    OP stated it's German, I'd expect that to mean the receiver, and not a SW receiver. Get it welded and refinished before it spreads. The welding process may have hardened the steel. The good news is that it should be easy to repair, one of the benefits of steel receivers.
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    It's used for SOT, yes it's German and fairly new.

    We had to send it to the smith for repair as it was needed for service asap, I'll get pix next time.

    But while in repair, we found that the bolt drop test failed when the butt stock was on (does not fail when no stock).

    Leads me to believe (I'm no expert here) that some debris got into the receiver or receiver warped, as the receiver cycled back it bulged the sides and caused the eventual cracks. These cracks are minor but we had them re-welded and we'll have it tested again soon.

    Ran a long blade file along the receiver inner rails to smooth it out and hand pryed the reciever a bit open and now bolt drop test passes.


    Let me know if there is anything I should check for in particular.

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    Quote Originally Posted by VX666 View Post
    It's used for SOT, yes it's German and fairly new.

    We had to send it to the smith for repair as it was needed for service asap, I'll get pix next time.

    But while in repair, we found that the bolt drop test failed when the butt stock was on (does not fail when no stock).

    Leads me to believe (I'm no expert here) that some debris got into the receiver or receiver warped, as the receiver cycled back it bulged the sides and caused the eventual cracks. These cracks are minor but we had them re-welded and we'll have it tested again soon.

    Ran a long blade file along the receiver inner rails to smooth it out and hand pryed the reciever a bit open and now bolt drop test passes.


    Let me know if there is anything I should check for in particular.
    The 40 cal and 10mm mp5's were only made for about 8 years and that run ended around 20 years or so ago so it's not a new gun by any stretch.

    This is not a reweld? Are you 100% sure?
    If not this seems seems super odd and I cant imagine any scenario like you mention causing this.
    I have never heard of a receiver cracking like you describe on both sides under any circumstances.

    Seem like the obvious place where a weld crack would develop especially where you describe it happening as that is a common area on the receiver to be cut during demil.

    Please post pics when you get the gun back. I would be very interested to see the repair.

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    Quote Originally Posted by AviatorDave View Post
    OP stated it's German, I'd expect that to mean the receiver, and not a SW receiver. Get it welded and refinished before it spreads. The welding process may have hardened the steel. The good news is that it should be easy to repair, one of the benefits of steel receivers.
    Well they don't make "new" .40 or 10's anymore...so that's a stretch..

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