I have been reloading for years but am new to casting my own bullets. With the help of my son, I have purchased and setup my equipment. We started out casting 45ACP using a lead alloy of unknown origins. The bullets seemed to cast well and after a few rejects we began to turn out nice looking bullets. The problem arose when we began to size the bullets. We noticed that although the mold was .452 and the sizing die was .451 most of the bullets went through the die with little or no resistance. I loaded a batch of 100 of these bullets and fired them against a batch of "store bought lead" reloads using the same powder formula. The "store bought" rounds performed well but the newly cast rounds tumbled like crazy. Any ideas as to what is causing these bullets to shrink?

I found out years ago when casting roundball for muzzleloaders that the final diameter of the bullet is a product of the bullet alloy. A mold is cut to provide a specific final diameter for a specific metal. In the case of my roundball molds, they are cut to cast pure lead to the stated diameter. Wheelweight balls from the same molds will be smaller. You need to add more lead to your alloy -- whatever it is.