I don't know from my own experience but ...... I read a review of EH Harrison's NRA book Cast Bullets today. I googled the author/title to find it. The reviewer was reviewing many different titles on the topic.Here
is the webpage, he said that the Harrison book had good info if you ignore the using of Dacron filler for reloading cast bullets. He said it was found to be unsafe. I hope someone with more knowledge on the topic comes along as he didn't elaborate. Just a heads up.