Just thinking about this the other day. I leave WD40 in my barrels after I clean them and never swab it out before I begin shooting. I doubt that WD40 is a compressible liquid so I began wondering what happens with the first bullet fired. I realize that most of it is expelled by the gases preceding the bullet but there has to be some that gets "ironed" into the rifling by the bullet. Actually if you compress it enough you can probably get it to "diesel" in the bore. So I was wondering if this residual oil left in the barrel could be the cause of some of the pitting in the bore that we normally attribute to fire cracking? Would be nice to hear a manufacturers take on this.