It’s FDE cerakoted. It was working fine for the first 60 rounds, a little dirty now, but still. I think the feed ramps might be slightly out of spec or misaligned or something. In tests the bolt head can briefly catch on it as I push it in. I tried the BCG I was using in a different gun and it didn’t appear to have that issue. We’re talking about a fraction of a millimeter here. Perhaps, when a new round is stripped off and pushed into the chamber, if the buffer doesn’t put quite enough energy on the bolt carrier the round slows it up enough for it to not be able to make it past the feed ramps. Just kinda gets stuck.
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I had to file the feed ramps on my last build. It was causing FTF issues. But it doesn’t explain why the bolt is freezing up. When you break it all down, are there any points of wear that can be seen? There could be some misalignment in the BCG with the chamber.
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Did anyone wonder how the hammer fell without being in full battery? Is there a clue there? Is it even possible?
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On an ar, depending on the position of the BCG, the hammer can still fall, it just can’t make contact with the firing pin unless the bolt assembly is fully rotated/locked in place.
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I cannot make out any obvious points of wear.
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That is a valid question… honestly I don’t actually remember if there was a “click” or just a dead trigger. My memory isn’t good enough I guess 
You might be right
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I dunno that I would be considering making modifications to the platform until I had done all the cheap stuff first. Scrubby the BCG and bolt.
Take a good look at your barrel side locking lugs and look for a shiny spot as well as one on your bolt lugs.
You made mention that the bolt is not “smooth”. Other than a clean and lube if the problem persists you could try some abrasive grease such as “Gunslick” or “Flitz” (between you me and the fence post, I did a final trigger job with Crest tooth paste once) on the bolt lugs and just cycle the he!! out of it, much more fun to shoot it though. It doesn’t take much to smooth things out and when it is totally smooth you will need to de grease the bolt and the locking lugs on the barrel with brake/carb cleaner to get all the abrasive out and then oil the he!! out of it.
I’m still leaning towards out of spec ammo as you said the case got stuck but then you mentioned that you probably fired them afterwards?!?!?
Does your barrel have a bunch of un burnt powder in it when you look down it or small yellow or grey “balls”? Some ammo mfg’s will use “filler” in straight walled cases to keep the powder level up when using a fast burning powder. (I know the 300 BO is not technically a straight walled case but it comes pretty close.) If that is the case it could be a bit of that non burnable filler got sucked back into the chamber which stopped the round from seating.
Just throwing ideas at the wall to explain two instances in 90 rounds that self corrected. Take good notes the next time you are out at the range if it should happen again. Recover the round for examination, take pics of any abnormalities. Note the number of rounds fired, which mag and which round # in the mag it was. The more info the better.
Cheers,
Craig6
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Thanks, all good advice 
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