ClapperZapper is correct. It is very simple to calculate the volume of your bullet, knowing how much it weighs when cast of pure lead. But there are so many alloys used as bullet metal, that you would need to have an assay done to know the exact composition of your unknown alloy.

But you may find what you are looking for here:

http://castboolits.gunloads.com/showthread.php?105952-Lead-alloy-calculators