I stopped using the plastic shot cups, of all types, with Black Powder.
This was because of the large amount of melted, now, black plastic I was removing from the bores during washing/cleaning.
I was using 60 grs of 2 fg powder which worked very well. But, even that amount of powder made the barrels very hot with only a few shots. Hot enough to melt the plastic wads during its short time in the bore.
My best,
Blair