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?
I have not looked at the links yet, but all the bullet swaging dies I have seen squirt the excess lead out a hole in the nose, and this sprue is then broken off....