I've been using GM upper engine cleaner for bore solvent for a long time now. Use it in all kind of firearms. A bit of that stuff on a rag and then a good bronze brush and in a few back-and-forth trips with the cleaning rod my barrels come clean. Many times I'll wrap a flanel patch arround a bronze brush and it helps hold the solvent at the brush where I want it. Spend all the money you want on the high powered cleaning solvents available, but youll be hard pressed to do better than GM upper engine cleaner. It isn't a serious copper removing solvent, but that isn't an issue for shotguns anyway.
BP
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