AR-15 failing to go into battery

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Perplexed

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I have a newly built AR-15 pistol with side charger with me at the range, and I’m having an issue with it. If I insert a loaded aluminum USGI magazine into the AR and hit the bolt release, the bolt will strip a round and chamber it, but the bolt itself will not go fully into battery. If I remove the magazine and pull back on the charging handle and release it, the bolt goes fully into battery and will fire normally. I tried a known good bolt from another AR, and experienced the same issue. I also tried another, known good magazine, and still had the same issue. If I load two rounds into a magazine and charge the AR, I have the battery issue, and if I remove the magazine and charge the AR again, the bolt chambers fine. When I reinsert the magazine with one round in it, and fire the AR, the bolt ejects the spent round and chambers the next round but will not go fully into battery. Two different bolts and two different magazines, all of which work just fine in my other AR.

What do you folks think could be the issue with the new AR?
 

O4L

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I've never messed with a side charging upper but trying it on a different lower sounds like a good idea as does changing the buffer spring.
 

HiredHand

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Could a carrier key and gas tube misalignment be part of the issue? I suppose you could have an out of spec bolt stop or magazine catch that’s causing the bolt catch or magazine to drag on the bolt carrier, too.

I’d pick a private place to swap lowers and just do a function check without live ammunition.
 

panayoti

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Check the case rim where the extractor grabs it for burrs or deformation this may indicate a tight or short chamber. A chamber finish reamer can get it in spec. Did you check the chamber with headspace gauges?
 

Perplexed

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Thanks for the responses, folks. I did take several springs and buffers with the idea of trying different ones, but the one-piece spring/buffer that I’d installed wouldn’t come out, so I’ll need to take apart the lower to extract it. No time today, though. I’ll try the suggestion of a known good lower, too. I don’t have any headspace gauges, but should it come to that, I’ll pick up one.
 

swampratt

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I have seen where the bolt will rub on the magazine lips.
I have a tight ar15 rifle that the magazines have to be bumped hard on the bottom to fully seat and another one that simply pushing them into place secures them.
This is with the bolt closed of course.
 

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