Quote History Quoted:
Per BCM, make sure you have the o-ring installed under the extractor for the 11.5 uppers.
I had a similar issue.
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The O ring has nothing to do with failing to chamber/feed.
I would check everything over for gas leaks, gas key and gas block primarily. Make sure your bolt is retracting all the way when cycling by hand. Also check out the lower, make sure the hammer can move through its full range, there's nothing under the trigger. Pull out the buffer and spring, make sure everything looks good.
Might be worth trying another magazine if you don't find anything else going on. Whatever you do, don't pitch those magazines, or send them back to Magpul... there are many who desire them, so just sell them on the EE.