I cant prove it yet, but I think the problem with the typical low priced rifled slugs is that the raised 'rifling' vanes and soft lead composition and undersized O.D. is prone to not engaging the rifling of the barrel well (it sure wouldnt work well in a typical rifle and these are really 'bore' rifles).
I think that a harder alloy, smooth sided slug that is at least bore dia. should shoot well, and I am in search of info on as cast dia.s from such, if anyone has it.
Short of that, try some in yours and brush the bore well after 3 to 5 shots and see what you get.