Is it hollow? The deep-running downrigger weights I've used have all been cast and are solid. Cheap to make since you loose alot. A hollow casting is a different bird. If you are looking for weight, why make it hollow? If it is hollow, then size has something to do with it.
unfinished Cannon ball?
When crew-served smoothbore cannon were scaled down to a size suitable for use by a single soldier, cast iron was still used for a time, but ultimately proved too light in small calibers. A naturally-occurring mineral, galena, could be processed with primitive smelting equipment to yield metallic lead. Lead can be melted over a stove or campfire with simple gear and poured into a mould, or easily cold-worked to produce a desired shape.
Lead is up to 60 percent denser than cast iron, so a lead ball weighs much more than one made of iron. In addition, the improved weight increased the range at which the projectile was effective.
Lead round balls fired through smooth bores were the military standard for years.
Daryl