I assume you put a piece of lead alloy in the lower die then squish it into shape, right? I would also guess that the slug would have to be precisely the correct weight before swaging. A slug that was too small would not fill out and one too large would not compress fully, right? How are the preforms precisely sized before swaging? Thanks.
The pieces of lead are usually cut from a coil of lead wire using a simple hole through which to push the lead wire, which contacts a "stop", and a leaver is pushed down to cut the core. The dies to swage the bullet has a "bleed" hole where the small amount of excess lead is forced out.