I had one failure to fire, it left a tiny indentation on the primer, Ram ammo 55grain FMJ, reloaded it into the mag and it fired the next time. Went on to fire another 100 rounds without a repeat of the incident. Hopefully just a fluke, was shooting at paper, nephew shooting the rifle. Cleaned BCG extensively when I got home, quite a bit of carbon build up around the bolt and the firing pin bore.

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Grover Oathkeeper

Every round chambered will leave a very slight indentation on the primer.
It's due to the fact that the AR has a free floating firing pin, every time the bolt goes forward the pin will give a light strike to the primer.

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Hold my beer and watch this!

If its only one round that FTF it was probably a fluke. perhaps a piece of carbon got between the fireing pin and the primer, or maybe it was just the round itself?? Just keep "oneyeopn" on it

Of course, this does mean that you are no longer welcome in the "My Sport is perfect" club and your membership has been changed to the "My Sport is awesome" club! ;^)

Or, we could just blame it on the ammo!