Hmmm... Tennis balls filled with cement. Hmmm... may work for my piece. Didn't know if the cement would shatter when shot? I experimented with cutting a tennis ball in half, then using the 2 half's to encapsulate
the smaller dia ball. 1/2 a tennis ball was great for windage but tended to rotate when loaded so tried both halfs. Turns out not all tennis balls are equal in respect to inside diamater. Some hand fitting required.
The tennis balls do work great for cleaning. Tie a long string to them, ram them home and pull out.