That Montana Extreme Copper killer is potent stuff. I can't use it indoors, it's harsh. :) I did this same test, but over a 20 day period. The copper killer removed ever so slightly more copper than the Hoppes #9. Just barely more, but Hoppes #9 was the clear second place winner over the others I've tested. I'd choose Hoppes over the Copper Killer, just because of the smell. :)