help please: .trusty 45 UMP -> 9mm jamomatic
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Thread: help please: .trusty 45 UMP -> 9mm jamomatic

  1. #1
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    Default help please: .trusty 45 UMP -> 9mm jamomatic

    I have a date code AA USC that I bought new way back when you could only get them in grey. Bought the HDPS stock block, died it black, SBR'd it etc. about 15 years ago. It has several thousand rounds through it, some suppressed, and has always run like the fine German machine that it is.

    In August, I bought a 9mm conversion kit from hkparts, with two German magazines. When I installed the barrel, I swapped out the ejector that came in the conversion kit for the stock one. Took it out to the range and every 7-10 rounds there is a failure to eject. This was with Remington UMC 115gr (I know, it's weak) and AE 147 gr.

    @hk21man posted in these forums recently that he has fixed this problem in a couple of these guns by replacing the extractors with a new German one, so I ordered an HK 9mm extractor from MGW and tried that. I was optimistic that this would help, see the photo of the new one on top and the original one on the bottom:

    help please:  .trusty 45 UMP -> 9mm jamomatic-img_1874.jpg

    Unfortunately, today I shot some MEN 124 gr, and PP 124 gr after installing the new extractor and the problem persists.

    Ejection, when it does occur, is very anemic: the cases fly 1-2 feet away. My USPC swallows the same ammo and flings the cases a good ten feet. I would appreciate any suggestions that help to get this beast running.

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    I had the same problems you are describing. My UMP in .45 worked 100% but when I changed over to 9mm, using US made parts it was not so great. I tried the swapping out ejectors with an HK factory one and it worked a little better but nowhere near as well as the .45. The only thing that ended up getting everything working for my UMP in 9mm was I put an original factory HK 9mm UMP bolt (complete factory HK not a copy, modified by TommyB to fit Omega UMP receiver) when I put that in my 9mm UMP it ran just as well as it did in .45 100% perfect. I don't know what makes the German bolt work and the USA made bolts jam but the proof is in the firing. Now finding a HK made UMP 9mm bolt is going to be hard and pricy when you do find one but it was more than worth it to me personally to have my UMP in 9mm it's a very nice gun in that caliber much smoother operation than the .45.
    Last edited by GregAB; 09-14-2019 at 02:16 PM.

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    Quote Originally Posted by GregAB View Post
    The only thing that ended up getting everything working for my UMP in 9mm was I put an original factory HK 9mm UMP bolt (complete factory HK not a copy, modified by TommyB to fit Omega UMP receiver) when I put that in my 9mm UMP it ran just as well as it did in .45 100% perfect. I don't know what makes the German bolt work and the USA made bolts jam but the proof is in the firing. Now finding a HK made UMP 9mm bolt is going to be hard and pricy when you do find one but it was more than worth it to me personally to have my UMP in 9mm it's a very nice gun in that caliber much smoother operation than the .45.
    Thanks for sharing your experience. These things are just brain-dead simple. I have never had any armorer training, but it seems to me the only things that can go wrong are
    • weak ammo
    • bolt is too heavy
    • chamber is rough/dirty
    • bad extractor
    • bad ejector

    I suppose, given that the bolts need to be slightly thinner than the full auto ones in order to fit in a USC upper, if the bolt is a little too wide you might get some friction slowing things down. There does not seem to be any more drag to me when sliding the 9mm bolt back and forth in the stripped upper compared to the .45. Assuming my new German extractor is fine, and the ejector is good, that leaves a heavy bolt or a nasty chamber. If you get a chance and have the ability, would you mind weighing your bolt sometime? I'm gonna remove my barrel this weekend and take a good look at the chamber...

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  5. #4
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    UMP HK 9mm bolt Weight is 25 oz.

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    Hmm. UMP HKParts 9mm Bolt weight is 23.2 oz, so my bolt is 10% lighter. Interestingly, Stock USC HK .45ACP bolt weight is 25.2 oz. Pressures and energies of .45ACP are higher than 9mm. I wonder if 10% is enough to have the bolt unlock early before peak chamber pressure is reached. If so, then that might explain things, If not, then my bolt should be moving faster than yours would in my rifle.

    Another matter of concern is that I just noticed HKParts did not ship me a complete firing pin assembly, even though the product description for HKP-02287 clearly states "firing pin assembly". (complete assembly is at the right of the pic, parts received from HKParts at the left)
    help please:  .trusty 45 UMP -> 9mm jamomatic-image.jpg

    It had been so long since I took the bugger out of my .45ACP bolt that I did not notice it when I disassembled and cleaned the new 9mm one. This could also be part of the problem, and at least is a safety concern because the bushing and spring prevents slam fires during operation and provides a drop safety. I'll reach out to HKParts and see what they say. In the meantime, I might go back to the range and try again using my old firing pin assembly. it adds a whole half ounce to the bolt...

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    My 9mm UMP bolt was weighed complete not stripped and it's a tungsten filled bolt, if that's any help. The other thing I remember reading about was some of the 9mm US made UMP barrels had very tight chambers? If you put a round in the chamber it would stick and have to be pulled out, on my barrel (HK factory UMP 9mm barrel) the 9mm rounds just fall back out if I turn the barrel muzzle up. The other thing is I got my bolt complete with recoil assembly that came out of a UMP 9mm police trade in, I'm not sure if the 9mm spring is a different strength than the .45?
    So, basically I originally built my UMP .45 into a 9mm using US made barrel, bolt, ejector etc., it did not function very well with the US made parts. After switching out as many parts as could with HK UMP original parts... the 9mm UMP runs great. I'm building a G36C using a TommyB. receiver but I'm only using original HK parts for that build as I find the HK parts just plain work and yes, I know it's some kinda German space magic and it's $$$ secret squirrel stuff but it works!
    Last edited by GregAB; 09-14-2019 at 08:53 PM.

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    I also weighed mine fully assembled and took them apart after. Your point about something potentially being wrong with the chamber is well taken, but if I understand you correctly, I don't think the test you mention is useful: presumably you're doing it with an unfired cartridge. After firing, the case expands to match the chamber dimension and is harder to remove. If the chamber was that small, seems like it might lead to unsafe pressures. maybe the chambers are rough or are need of polishing and that makes the case hard to exrtact. everything I've seen indicates that there is one recoil spring design for all calibers.

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    Just curious: what recoil rod/spring assembly are you using?

    - Factory USC (german)
    - Factory UMP (german)
    - HKParts USA

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    I mention the chamber test (yes with an unfired cartridge) because there was a discussion on this board or one of the other forums and they were also having the same problems with the 9mm UMP as we experienced. Several mentioned that an unfired round got stuck in the chamber of their US made 9mm barrels and if that was happening to an unfired round it had to be worse after firing right with the expansion like you said?

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    I’d like to add, while I purchase from HKParts, I’ll only buy German made products. When they first came out with MR556/416 accessories they were horribly finished products. My impression is that their manufacturer QC lacks any rigor, unlike that Teutonic witchcraft QC across the pond over at HK mothership.
    "Just because you're paranoid doesn’t mean they're not after you!"

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