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Discussion Starter #1
I recently did the USC to UMP conversion. The USC magazines work perfectly fine. I have purchased three different UMP mags and im having the same problem with all three. I have a factory UMP mag, military mag, and a US made mag. In all three the shells bind in the magazine the the back of the shells dont settle into the top of the mag lips. This causes the bolt to slide over the shell.

I have yet to fire the gun, but the difference in how easily the USC mags and the UMP mags feed concerns me.

Any ideas?
 

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Who did your conversion?
 

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Discussion Starter #3
I did. Ive inspected the placement of the UMP mags and the USC mags when they are inserted, and they are in the exact same place. Only difference is the shells of the USC mags hit the top of the lips 100% of the time. While the UMP mags its about 80% of the time. The rest of the time the back of the shell just doesnt come up all the way.

Perhaps it was something i did wrong, but i cannot see anything that could be causing this.
 

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Did you open up the bottom part of the receiver enough for the mags to fully insert?

I had to open mine up a hair more than I originally did to get it to fit and function.
 

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Do the shells come up to the top of the feed lips when the mags are out of the gun? If the problem only occurs after the UMP mags are inserted into the gun, it's a conversion problem. Also, did you open up the hooks on the front of the UMP lower? It sounds like either the lower isn't sitting right, or the under side of the receiver isn't opened up enough.
 

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Discussion Starter #6
Pretty sure. The edges of the magazine sit flush with the bottom of the upper. Same as the usc mags. I may take some more material off even though I just don't see that being a problem. Before I do I wanna exhaust every other option.
 

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Discussion Starter #7
Even out of the gun I can push down on the back of the shell and it won't pop back up like they will in the USC mags. It seems maybe the springs are not strong enough to straighten out the shells. It's just weird that it's happening on three different makes of magazines.

I did however get all three springs from hkparts.
 

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First, I want to point out I have not done a conversion. However, my understanding is you have to mill or open up the Magwell to accept the larger UMP magazines. There is a good video on YouTube showing how to open it up. I would watch that and see if that helps.
 

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Even out of the gun I can push down on the back of the shell and it won't pop back up like they will in the USC mags. It seems maybe the springs are not strong enough to straighten out the shells. It's just weird that it's happening on three different makes of magazines.

I did however get all three springs from hkparts.
So you are using replacement springs in all of the mags? What happens if you use the factory UMP mag spring?
 

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Discussion Starter #10
No. The factory mag I got came complete for hkparts.

For anyone that has these mags, is it normal for the shells not to spring 100% up to the top of the lip? Perhaps the bolt and mag need to be closer in my gun?
 

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No it's not normal.

There appears to be a lot of contradictory information in this thread.

First, are you having problems with the shells getting jammed and not reaching the feed lips with the mags completely out of the gun? If so, it's a mag or ammo problem, not a gun problem. If the mags work fine and spring back to the top outside of the gun, it's a gun problem.

Second, what ammo are you using? Does the same problem happen with the mags out of the gun with different ammo? If it doesn't happen with different ammo, it's an ammo problem. If it does, it's a mag problem, although I can't fathom how a completely unmodified factory UMP mag would have a problem with regular factory ball ammo.

Third, if the problem doesn't happen in the mags outside of the gun, and only when inserted in the gun, it's a gun problem. Either the receiver isn't opened enough, or maybe the hooks on the lower aren't correctly opened up and the lower is sitting too low.

What UMP lower conversion, are you using? UMP rear stub weld, or stock block? If a stock block, who made it?

If you still have problems and can't figure it out, I'd recommend sending it to a gunsmith familiar with the UMP conversions. Good luck.
 
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