I guess the obvious question would be. are you shooting factory or reloaded brass?? The debris in the chamber had to come from somewhere and unless it was left over from test firing by the manufacturer, it would almost have to be from recently trimmed brass that was not chamfered or beveled...
You might chuck the chamber brush in a section of cleaning rod with some lubricant on the brush and use a drill motor to spin the brush in the chamber for about 20-30 seconds in an attempt to knock out the debris that is there....The .308 does develop some serious chamber pressure, but any debris should not have been pressed into the chamber walls....
Anything that decreases the chamber dimensions will cause hard extraction and the scratches on the cases may be the cause of the particles in the chamber from shaving the surfaces, in which case the upper needs to be sent back to the manufacturer for evaluation and correction...
