The one time I shot black powder with plastic wads, I was totally unprepared for the mess afterward. I just got it out with a bronze brush. It took a long while, not because the stuff was tough or welded on, but like there were layers and layers of plastic, and each had to come out in sequence. If you use Claybuster wads, you might already have some plastic solvent around. If not, I'd forego the expense and brush it out.