I don't think those are pure lead. I read somewhere they put a couple of percent antimony in the alloy. Not as hard as cast but not as soft as pure lead.
They don't necessarily lead a bore. The condition of the bore and the sizing are key. With a revolver, I usually find the least leading with bullets 0.002" over groove diameter rather than the usual 0.001" over groove, assuming that fits the chamber throats well.
OP, try weighing a number of the bullets. For target shooting with a revolver, I try to keep them within about 3 grains of each other. In this case, though, you are looking to see if the caster wasn't getting the mold closed completely due to debris. You are also looking to see if you can detect different weights of roughly equal quantity, indicating which cavity threw which ones, so you can sort them by mold cavity to identify the most accurate ones.