Buy some 0000 (super fine) steel wool, wrap it around a 20 gauge brush (12 gauge with steel wool fits too tight) and put it on a drill. Give it about a 30 second spin, run a mop through the barrel and you're done. Much better results than a $40 flex hone brush and between the wool and brush, you're spending maybe $10. I had an 870 with a rough chamber and a lot of extraction problems a while back and this would have been some really useful information for me to have had back then so I thought I'd share.