I shoot BP shotgun ctgs all the time. In the older guns with frosty or pitted bores cleanup takes a little while, but with new shiny bores it's just a quart of cold water down each tube followed by a couple of patches, a little heat to dry things out (putting the barrels in the sun a few minutes works well) then a swipe with Ballistol. I keep plastic away from those barrels, that does make a mess!