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Thread: Upper/Lower Fitment Issue

  1. #11
    Join Date
    Aug 2011
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    Curious to the make of lower?

  2. #12
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    Jul 2011
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    Peoria, Illinois
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    AKDoug may have hit the nail on the head. I have an Aero upper and lower that are likely just a hair out with one another. They mate up with other combinations of uppers and lowers however. The thin layer of cerakote may be contributing to the issue. I used a Dremel to remove .005 or so off of the rear pin so I wouldn't need to hammer it in.
    Last edited by Torch-36; 12-09-18 at 15:45.

  3. #13
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    Jan 2008
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    AZ-Waging jihad against crappy AR's.
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    If you have tried it with 2 different lowers (although you left out some details) and you think there is an issue, then contact LMT as per the site rules.



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  4. #14
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    To the OP, are you attempting the front pin hinged or flat? I bought an MRP upper last year from Fin which shipped directly from LMT complete. This upper is extremely (uncomfortably) tight on my newest and unused non-MARS lower, and fits very close fit on lowers with more time. LMT is machining pads into the upper which I believe by design are supposed to tension the pair a bit so the top and lower half are tight. Shooting teams would do something similar by taking a piece of shell casing and shimming between the two. The pads are about .75" back from the front pin, and are ~.005 thick. Pic attached:
    IMG_20181227_225105883.jpg

  5. #15
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    May 2012
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    St. Louis, MO
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    Lowers are both Spikes (I know, I know...). Neither have had issues with other uppers (Vltor and BCM). Front pin is still tight, but it does go through with some persuasion and rifle functions normally. Tight is indeed good.

  6. #16
    Join Date
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    Quote Originally Posted by Achilles11B View Post
    Neither have had issues with other uppers (Vltor and BCM). Front pin is still tight, but it does go through with some persuasion and rifle functions normally. Tight is indeed good.
    I've experienced tight fits with an occasional BCM UR here and there, but nothing like the MRP and that newest LMT lower. I do believe they would wear in and be fine, but the pads may explain the difference between your standard uppers (Vltor and BCM) and this MRP upper. It was just a data point I've not seen anyone else mention.

  7. #17
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  8. #18
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    Jun 2011
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    Sounds like a good problem to have! In all seriousness, I've heard these are tight from many people. And function well.

  9. #19
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    Apr 2014
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    What kind of lower?


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  10. #20
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    Jan 2019
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    I've seen that happen when the buffer tube has been installed one too many turns. The lower is causing the upper to sit forward just a hair because the upper is hitting the end of the buffer tube. Try backing the tube out one turn, just make sure that it still captures the buffer retainer.

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