Oh boy Dan,I'd cringe also!.For what it's worth,I don't have a BP shot gun,but I'd sure'nuff try to find some way to cut or fabricate some shotshell plastic ta work for steelshot down the bore.
I live in Minn. and usta hunt quackers avidly,I'm old enough to remember the change from lead to steel.An it sure rasied havic with our 870's,had ta have'm honed out a bit ta group proper.(after the bulged barrel!)
Might wanna check out the properties of that Bismuth stuff.
Wish I could help more!?? good luck!