I have only been casting for a short time. I have read much and tried to learn from many here and other publications.
So I have a few molds. I have good success with certain molds. But a couple 158 & 170 SWCs and a couple 309 rifle molds, I fought getting complete fills. Using COWW with 2% tin added. So I'd crank the heat up and preheat the molds. Trying not to get frustrated, I about accepted that I would have a certain amount of rejects. BTW, I use a dipper with a Lee 20# magnum pot
A coupe weekends ago, I had a couple new GC molds that I wanted to get going with. Same thing started happening. Then it finally dawned on me. When I started seeing the non-fillings, I would crank the heat up start pouring faster. In my mind I thought the lead was cooling too fast to fill the void completely. The light bulb moment was when I realized that by pouring faster, air was getting trapped in the cavity. That weekend I was able to cast 650-700 varieties of bullet with very little rejects.
I knew venting was important, but now realize that air needs time to escape. I will probably reread something that told me that before.