Sproul - it's possible that it may be lead. I was shooting my Marlin Cowboy a couple weeks ago using hard cast lead bullets with a pretty stout load. I used Hoppe's Elite Lead and Copper fouling remover. Scrubbed the bore with a wet patch, let it set a bit then worked a brass brush through the bore. It took quite a while to get all the lead out of the bore. You could see the fine slivers of lead on the patches. And being that Maxis are a lot softer, lead fouling is a good possibility.