Why does this keeping happening to my USC/UMP conversion - Page 3
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Thread: Why does this keeping happening to my USC/UMP conversion

  1. #21
    Gets the Shakes if No HK Contact in 24 Hour Period

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    Quote Originally Posted by BigBoss0311 View Post
    I would also start with cleaning and lubing your weapon. If Mr. Bostic did it, I would leave that for last to check off. I will ask if you bought new or used mags and how the springs in them are.
    My mags were brand new when I purchased them. and I think I'm going to lube the BCG and receiver like @Oicani told me to do.

  2. #22
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    Mine does this with the US mags from HKparts. I ended up swapping springs, followers etc and found that the problem stayed with the particular mag bodies. I also found that when left loaded and not in the gun that the feed lips would widen slightly. I think it just comes down to the US made mag bodies don't have the reinforcement or something in the feed lips and they let the round out too easily. I have never had a jam with the HK original mags.

  3. #23
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    Quote Originally Posted by Techspy View Post
    Mine does this with the US mags from HKparts. I ended up swapping springs, followers etc and found that the problem stayed with the particular mag bodies. I also found that when left loaded and not in the gun that the feed lips would widen slightly. I think it just comes down to the US made mag bodies don't have the reinforcement or something in the feed lips and they let the round out too easily. I have never had a jam with the HK original mags.
    Those US made mags are garbage. I have one that didn't work after like 4 uses. The lips can't even hold in more than 10 rounds.


    The newer ones have metal reinforced ones may be better.

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  5. #24
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    yes, get the actual HK mags if you aren't using them.
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  6. #25
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    Mine has the same problem and in my searching it seems to be a common problem. I think ive figure out whats wrong, but dont have a solution. Is your bolt sliding over top of the shell?

    The USC and UMP bolts are different. There is a cutout in the USC bolt to fit the feed ramp in the USC barrel. The place where the bolt makes contact with the back of the shell in the UMP is flat without a cutout, so it makes contact with the center, and therefore highest point of the back of the shell. The USC bolts cutout means it makes contact on the sides of the back of the shell, which are lower than the center.

    Now the reason this matters is because the UMP magazines dont push the back of the shells up as high as the USC mags. The shells in the UMP mags sit lower, while the contact point of the USC bolt is higher. These combined cause the problem with the USC bolt sliding over the shells in the UMP mags.

    The solution is to somehow get the shells in the UMP mags to sit higher. How? I havnt figured it out.

  7. #26
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  8. #27
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    Quote Originally Posted by MasterBlaster View Post
    Yours doesn't look too bad, but a good way to check is to take a mag without the follower in it and see if you can get the body up in through the hole.
    Here is what mine looks like now.

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    Wow, you took away a lot of material, to the point where you can see the bottom of the feedramp... I did mine based on the info shared here, and verified fitment with both the UMP45 & UMP 9 magazine and I have not had any feeding issues with mine.

  9. #28
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    Are you using a USC bolt? On mine the UMP doublestack mags will bind and prevent the back of the casings to go all the way to the top of the magazine lips. The USC bolt with its cutout will then slide over the casing causing it to jam.

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