Hello, Grumpybear. Pure lead will retain true dia. of sizer die..that why this type of lead is recommended for slugging bores, etc.
lead which is alloyed with tin or antimony will be harder and have a bit of springback. I have Corbin swage dies..When I order, I specify dia. be based upon pure lead. My own alloy of fairly hard steriotype metal plus pure lead..about like the old Lyman #2 in hardness does have some springback.. about .0001" to .00015"...These are tenths of one thousand of an inch..not alot ot be concerned with.