The handle broke because the lead was too cold when you cut the sprue. Either pre-heat the mould, or just do the 2 cavities in front for a few bullets and then the front 4 for a few bullets and then maybe you can do all 6. I have said it before, I have at least 30 LEE 6 cavs and cast at least 2000 bullets a week for the last 7 years and have never broken an handle. The mould MUST be hot before cutting sprues.