I recently bought an auction lot out of a Denver area online auction that had 3 AR barrels that I am trying to identify. Two of the barrels are new and the third only shows signs of being installed on a receiver. A chamber cast shows that it is probably based off of the x39 family. Unfortunately the 35 year old cerosafe crumbled before we could get any other measurements. A new batch is on the way. Does anyone know anything about these barrels whose only markings are 6mm A.F. (1 in 12) N.R.A.