Monotype has a BHN close to 30 & should be 9% tin, 19% antimony and 72% lead. If you mix 4 parts pure lead to 1 part monotype you should have 1.8% tin and 3.8% antimony. If you mix 5 parts pure to 1 part monotype you should have 1.5% tin and 3.2% antimony. Either would provide an alloy for the 45 ACP.
I use a 50-50 mix of COWW & lead with fine results. Though my mix is tin starved [less than .5%] the bullets cast fine using a 750 degree setting. Hope this helps.
