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Thread: LR-308 Boltcatch issues checklist

  1. #1
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    LR-308 Boltcatch issues checklist

    I think most understand the 308 "AR-10'ish" guns are not as standardized as 5.56 and other small frame.

    So it's good to stay with the same vendor, or at least "pattern". Even with that you can find sneaky issues. The DPMS "LR308" is one of the most common ones, and is the defacto standard for clones. (IE: Not KAC, LMT, etc).

    I ran across one, and thought I'd share.

    TL;DR Human error is more common than machining errors

    Situation: New AeroPrecision LR308 complete Lower and Complete Upper; LR308 pattern BCG known to work with the LR308 and specifically, AP.

    All looks good, except the bolt catch does not catch on the bolt face. The BCG cannot go back far enough for it to do so, by 1/2". Otherwise functions correctly. (shoots good, even)

    The immediate thoughts are 'I bought a dog', "should have bought an AP BCG", etc.

    But some troubleshooting led to a completely different direction.

    - 1st suspect- Bad Bolt catch or AR-15 catch. Visual inspection confirmed it was a 308 catch, and the bcg would not go back anywhere near far enough for it to be a machining issue.

    - 2nd suspect- Short Buffer tube. Nope, the LR308 patterns use an M4 standard buffer tube. Which was confirmed. Which leads to...

    - 3rd (and common) suspect- Buffer too long. LR308 patterns use a shorter buffer than AR-15. (2.5" vs regular 3.5" M4 carbine buffer). Common error. But, nope, this one was a correct 308 2.5" buffer. This should be the 1st thing to check.

    So the immediate suspicion is the BCG... "should have got from the same mfg". But wait a minute, toolcraft 308 BCG's are pretty well respected, and this one look right. Specifically, it does not appear to be 1/2" off.

    - 4th Suspect (and again, common)- Spring is too long, wrong # coils, etc. So the test was removing the spring, reinsert the buffer, and there is plenty of room for the catch to catch the bolt face. So the spring was limiting the rearward travel.

    Some research led me to learn that LR308 springs are different from Armalite AR-10 springs are different from M4 223 pattern springs. (Length, and coils).

    And specifically, the spring in the 308 carbine I was working with was too long and had way too many coils. Thus was fully compressing and stopping the BCG sooner than it should.

    A quick call to AP, they fedex'd a correct 308 spring. Which addressed the issue.

    Not sure how it happened. Did they grab an M4 spring from the wrong bin at the factory? Did the store swap it out to address a problem, then replace it with the wrong one? Don't know.

    AP made it right, and it addressed the issue very quickly. AP was excellent to work with, even though I'm not sure it was their issue.

    So it was not mis-machined bolt catch, or BCG. Nor too short buffer tube, etc. Very simply wrong spring. Simple problems, not complex.

  2. #2
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    Nice troubleshooting guide. One caveat to add. I’m open to correction, but my understanding is the standard length AR-15 buffer is correct if using a Vltor RE10 extension. So don’t put the short buffer in the longer tube.
    “ When I comes to modern politics, I think the inverse of Hanlon's Razor applies...In other words, "Never attribute to stupidity that which is adequately explained by malice." - Kerplode

  3. #3
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    Quote Originally Posted by uffdaphil View Post
    Nice troubleshooting guide. One caveat to add. I’m open to correction, but my understanding is the standard length AR-15 buffer is correct if using a Vltor RE10 extension. So don’t put the short buffer in the longer tube.
    I don't pretend to know all the nuances, but there are multiple flavors and cases. And I believe you are right on the Vltor.

    Rifle length is completely different, but a bit more consistent best I can tell.

    The nuance on the 308 carbines is that one mfg (Armalite) uses a longer receiver extension/buffer tube (or kept it from the original AR-10?), which allows regular 3.5" M4 Buffers to be used. (But often with different weights, etc).

    While DPMS with the LR308 used M4 tubes. Which then required shorter (2.5") buffers and different springs. This is the most common case. If it's a clone that is not an Armalite AR-10, or a DPMS G3, then it's probably a DPMS LR308 pattern. (Brownells, Aero, PSA, etc). But confirm before ordering parts!

    Gen 3 DPMS is completely different entirely. Not sure about LMT MWS and KAC SR-25.

    The above makes me wonder if collapsed length on the Armalite AR-10's are 1" longer since they have longer receiver extensions?

  4. #4
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    LMT uses a longer receiver extension (vltor A5 length) and a standard size buffer. There are two different length DMR stocks for LMT, 5.56 and the slightly longer LM308. A SOPMOD on the LM308 won't collapse completely against the castle nut.

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