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Strange THERMOLD plastic magazine problem

bpostbpost Member Posts: 32,664 ✭✭✭✭
in Ask the Experts
I have several AR platform rifles all will take P-Mag's and metal mags just fine.  One build, a 16" 5.56 will not hold in the THERMOLD magazines.  I can knock the mag into the well on the table, some will stick then, others will not hold no matter what I do.  I changed the mag release catch and measured from bolt to mag well at several points all are within a couple of thousandths.  The THERMOLD mags will lock into the other AR's and be somewhat reliable with failure to hold the bolt open the most common failure mode on all rifles. 
Anyone else having this issue?  Anyone know how to solve the issue with the rifle that refuses to accept them?
I contacted THERMOLD several days ago supplying all the requested information.  I have not heard back from them yet.
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    rufe-snowrufe-snow Member Posts: 18,650 ✭✭✭
    Gorilla tape is your friend. The magazine well on your lower, is out of tolerance. Shim the mag bodies with the tape. Unfortunately these mags, probably won't work in your other lowers.
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    MobuckMobuck Member Posts: 13,793 ✭✭✭✭
    We use and like Thermold mags. Some require a good slap on the base to fully lock in but once engaged, the mags hold and function well enough. Maybe a bad batch or as mentioned, simply incompatible with that particular lower.
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    pip5255pip5255 Member Posts: 1,630 ✭✭✭
    tolerance issue
    just because you could doesn't mean you should
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    bpostbpost Member Posts: 32,664 ✭✭✭✭
    I have measured every thing I can get to in this receiver and it is within a thou or two of my others.   Do you think the issue is with the mags or the receiver? 
    The failure is where the magazine catch is, it does not seat into the thermold mag.  I changed the mag catch, no difference was seen. It works fine with metal mags.  Just these THERMOLD mags refuse to lock in.  Call me stumped.

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    MobuckMobuck Member Posts: 13,793 ✭✭✭✭
     "Just these THERMOLD mags refuse to lock in.  Call me stumped."

    Have you tried locking the bolt open when seating the magazine?
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    MgderfMgderf Member Posts: 907 ✭✭✭
    Take a set of calipers to the magazine release area on the steel /Pmags, then measure the Thermold.
    I'm betting that Thermold mag is ever so slightly thinner.
    Get rid of the Thermold brand mags and your troubles will end.
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    hillbillehillbille Member Posts: 14,183 ✭✭✭✭
    could the plastic mags be flexing??? just enough to not catch
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    bpostbpost Member Posts: 32,664 ✭✭✭✭
    See now thread, it is not a mag issue it was the mag release catch assembly need to be screwed in further.
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