well Im not so familiar with the american weight and volume units
but I dont think that american lead is heawier then european lead
iron have an density of 7,87 ( 7,87 kilo for one liter )
ok cast iron is a little lighter because of the carbon content
lead have an density of 11,337 ( 11,337 kilo for one liter )
so the difference is approximately 44 %
the units may vary , but the difference in weight is still 44 % for the same volume of the both materials