This video has stirred a little bit of controversy. If you search “chipped bolt lugs” you’re going to get a lot of ARFCOM and Reddit hits of people posting photos of damaged 9310 nitride bolts. And you’ll see a lot of responses from people claiming that the bolts were damaged by a canted barrel extension. The hypothesis is that the index pin or corresponding notch is out of spec, allowing the barrel assembly to rotate while the barrel nut is being torqued down. That explanation doesn’t pass the sniff test for me, though, and here’s why.
For starters, we overwhelming (if not exclusively) see this with 9310 nitride bolts. Were this a problem of out of spec extensions or receivers, the problem would show up at least as often with every type of bolt. Actually, you would expect C158 phosphate bolts to show up with this damage more often, because they are much more common, and, theoretically, are slightly less durable than 9310 bolts.
Second, it’s simply not logically possible for a misaligned extension to go unnoticed by the owner. The bolt would bind going into battery when installing the BCG in the upper receiver, and it would also bind when retracting the charging handle. I submit that a misalignment severe enough to cause this damage would bind the bolt so badly that it would be impossible to charge from the charging handle in the first place.
Third, the chips in the lugs are on the wrong side of the lug. If the extension were rotated by the barrel nut, it would be rotated clockwise (looking onward), and the lugs would be striking the extension on the opposite corners from where the chips are. What is notable though is that the chips ARE on the corners that are the last point of contact during unlocking. In other words, when the bolt rotates to unlock, those are the corners that are receiving the most abuse.