Both guns cleaned in the same way, shot at the same time in the same weather, similar number of rounds. The 12g barrels are almost spotless (after 150 rounds) -- the 20g barrels are /filthy/ -- not just the barrels in fact, the ejectors, and even beyond. There's crud everywhere, and flakes of unburnt powder -- both barrels too, so it's not just one 'bad' cartridge it seems.
Is it normal, or a 'bad batch' ?