pmarauder: I've never seen a trace ( not even one) of a bronze brush bristle anywhere in the receiver lug area, chamber or bore. This is one of the areas I regularly inspect with the borescope.
What I did find years ago when I was push/pulling the brush thru the bore, like sawing a piece of wood, was bristles breaking off, from the back and forth strokes, and laying in the corners of the rifling, where the lands meet the grooves. The cleaning patch (tight fitting & wet with solvent) would pass over the top of them & they remained. 'Til a bullet passed over them?
Since then, I push only, from chamber to muzzle & the brushes wear & start to feel lose, before any bristles break-off. I'm following the advice of Bartlein barrels ( shown on their webpage, FAQ) & remove the brush after each pass down the barrel. Time consuming, I know, but to me, worth it.
Push/pulling also gave me very short brush life, since the bristles are constantly being bent back & forth, no different than bending a piece of sheet metal a few times until it breaks.
Most of my rifles are switch-barrels, & everytime a barrel is removed/replaced, the receiver lug area is closely inspected, cleaned & re-lubed.
With my match chambers, I keep my brass over-all length's at .005" to .010" shorter than chamber length's, so never have a carbon ring in that area now.