Quote History Quoted:
Bolt lug recesses would be my guess.
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Ignore what I struck through. It is stupid.
That is one of my guesses, but the page also says to lightly grease the "bolt body including the lugs" and if you grease the lugs, some of it will xfer to the chamber extension. It would have been nice if they'd included a diagram.
I know this gets into the holy war over using grease in an AR action. In fact, the the lube guidance *is* using grease in an AR! Of course, this advocated putting down a sooper-thin layer with a small paintbrush. I can tell you from 1st-hand experience that you can put a sooper-thin layer of grease (Tetra) with a small paintbrush on the inside of the upper receiver, the insides and outsides of the BC & bolt, send 150+ rounds downrange and not have a problem.
Cleanup of the upper receiver and BCG is easy. Rubbing alcohol to wet a cotton cloth to break up the grease followed by a dry cloth to remove it seldom needs more than 2-3 repetitions before it is all clean.