AR15 Bolt Doesn't Lock Back After Last Round

FalArAk said:
It ran perfectly before, it is the exact same upper.

The new buffer tube is LMT(I like the indexing for the detent pin) and the old one was Geiselle.
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Right.. So your problem is somewhere with the lower. Are you running a H2 in your other lower?
I've also never seen an ambi bolt release. Learn something new everyday.
 
FalArAk said:
So bolt catch might have a spring that is too strong or something?
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If there's no mag inserted it shouldn't lock the bolt. I've seen out of spec mag catches before... friend had a smith M&P that was another variation of this problem. Who made the "factory" lower? It probably just had a shit grade lpk in it. Or you got a bad part?
 
drgrant said:
If there's no mag inserted it shouldn't lock the bolt. I've seen out of spec mag catches before... friend had a smith M&P that was another variation of this problem. Who made the "factory" lower? It probably just had a shit grade lpk in it. Or you got a bad part?
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It doesn't lock back without a mag, I was all excited and prematurely declared that it did and made a small mess :)

LMT factory ambi stripped lower.
 
What's that chunk of black crap at third from the right forward assist notch on the bolt carrier?
 
Mountain said:
What's that chunk of black crap at third from the right forward assist notch on the bolt carrier?
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Need an MS Paint diagram, not sure what you are referring to.

Edit: I see what you are talking about, those are the sand cuts..I would have to look what that thing is tomorrow, looks like it might be part of the dust cover maybe.
 
FalArAk said:
Need an MS Paint diagram, not sure what you are referring to.

Edit: I see what you are talking about, those are the sand cuts..I would have to look what that thing is tomorrow, looks like it might be part of the dust cover maybe.
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Maybe a burr on that dust cover. Anyway, small chunk of something where it doesn't belong.
 
It’s gotta be a fail with the ambi bolt catch somehow. I would take it apart and inspect. If a single sided bc has good spring tension and only lifts with an empty mag follower then I doubt you would see this happen with one shot on a full or partially full mag regardless of bcg, buffer, or spring.

Take a pic of your bcg locked back like this with the mag inserted.

C1291DEB-3FA9-4E91-9EBD-C7C893DF1867.jpeg
 
FalArAk said:
Using a different mag won't help me figure out the problem, the magazine works fine, just not with this upper for some reason.

I think if the mag wasn't in the right spot or bolt catch was not installed(by the factory) correctly, it wouldn't lock into the receiver or feed correctly etc

Mag release works as it should, feeds ammo fine, locks into lower fine.

I shot 60 rounds and replicated the problem every time.
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did you try turning off and back on again?
 
centermass181 said:
did you try turning off and back on again?
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Thats always the last resort.

One other thing I did different is lube. A small bottle of go juice or some shit came with the new trigger and I used it on the bolt. That stuff was more like a grease consistency.

I will wip it all off and check function today maybe.
 
Last edited:
Wiped all the grease off and it functioned fine. Used the 20 round, 30 round LaBelle and a P-Mag with the same PP M193

Wiping the grease off was the ONLY thing I did. How the hell does grease make that much of a difference, I didn't even put that much
Greased the BCG channels and the underside of the charging handle.

Thanks for all the help :)
 
FalArAk said:
Wiped all the grease off and it functioned fine. Used the 20 round, 30 round LaBelle and a P-Mag with the same PP M193

Wiping the grease off was the ONLY thing I did. How the hell does grease make that much of a difference, I didn't even put that much
Greased the BCG channels and the underside of the charging handle.

Thanks for all the help :)
Click to expand...
It can happen when you’ve got some tight tolerances or some things slightly out of spec. Then you’re going to be closer to the point of failure than you otherwise would be. If you run it a lot, it might sort of wear in/break in. But keep an eye on it when it gets dirty to see if you have problems.

I’ll also say that I’ve seen failures from excess grease but that was caused by using grease to quiet the spring down, and the grease gummed up when it got cold.
 
i had this problem on a lr308. everything looked right, if i opened the bolt by hand on a empty mag would hold but not while firing. it ended up being the mag catch. must have been machined wrong? try swapping the bolt catch with one you know works.
 
How much sloppy movement with pin/catch ? Too small of a pin, or slightly oversized hole in catch that slowly got bigger over use ?
Either that or your batteries are getting low. What kind are they.....😜😜[pot]
 

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