Okay, here's what I think. The lead sheets you're using are nearly pure lead. There may be a trace of tin in them, but no more than ½ %. That's why you have bullets that look like they do. You are WAY TOO COLD on both mold temp and lead temp. Casting bullets using pure lead is very difficult. You have to run that pot wide open, the hottest you can get it. Then you also have to preheat the mold very hot and keep it that way.
Best thing you can do if you want to keep using the sheet lead, is get some tin in that soft lead. 2% or better will lower the heat needed to get good bullets, and increase the fluidity of the lead and help fill-out of the bullets. Best way to do the tin add-in is to get some lead free solder. Get the 95-5 with the 5% being antimony. For a 20 pounder, you should put ½ pound of the lead-free solder in it. That would get you a 40-1 ratio. If you could find some bar solder, then toss a whole pound of it in. That would result in ½ pound of tin since most of it is 50-50.
Another solution is to use some of those wheelweights. I strongly advise AGAINST trying to melt wheelweights in your casting pot. Use a separate steel or cast iron,(no aluminum), pot over a heat source like a camp stove to melt the WW.
Also, be aware that some WW now are made of zinc. A small amount of zinc in bullet metal will ruin it. So, to keep the zince out you have to be careful when melting the WW. Zinc melts at 787 degrees. WW melt at 625 to 650 degrees F. The zinc WW will float on the molten lead IF the temp is below the high 700's. Then you can skim them off along with the steel clips and other crud.
The more people I meet, the more I love my dog
They're going to get their butts kicked over there this election. How come people can't spell and use words correctly?