I'm not referring to the soft powder deposits left down the entire length of the barrel, but the very, very hard shiny black deposits found in the first inch or so of rifling. Some reports refer to it as carbon based, others as aluminium oxide - presumably produced from the primer.

Anyone know of an actual chemical analysis ? Just curious.

C.R.U.D.

Well one authority here on AR was telling me that carbon is soft, like in a lead pencil.

Where diamonds come in I don't know, but one scribe here reckons that stuff gets that hard that one day it will "shatter" off, and out.