I do not recommend casting them out of WW type lead. Been there, done that. They are very hard to get down the barrel and are not consistent in weight and dia.
With that said, the last few Maxi Balls I cast a couple weeks ago was from pure lead. I melted down some Hornady Great Plains bullets which are pure lead. They turned out consistent in dia and fairly close in weight. They are easy to start down the barrel as well. Too easy to suit me, considering they go down too easy down the barrel and to the breech. It takes little pressure to seat them. I'm not sure as to if they will stay seated under certain circumstances. The same mold was used with WW lead and pure. That proves its the difference in lead that matters.
Also, the Maxi Balls cast out of pure lead are more accurate overall than those I cast from WW lead, especially at longer distances.