Art -
Back in the 50's (? 60's) a fellow named Harvey designed a series of moulds using a zinc washer (at the base of the bullet) wherein the washer was inserted into the mould and then the boolet was cast. The washer was to act as both sealing and scraping to eliminate leading. Protex the term associated with his product. You still see a few of the moulds around, made by Lyman and others.