Step 1 in making bullets is core cutting. For 6mm we start with 0.185” pure lead wire that
contains no more than 0.5% antimony. Higher amounts impede swaging and can be hard
on the dies. Mine is 0.1%, which is pretty much plain lead.  The wire is measured and cut to
18” lengths.  You get 25 cores per so to make 1,000 bullets I’ll need forty. Before
processing I wipe them with an acetone soaked rag. This just removes loose particles and
surface contaminants.  Each piece is straightened by gently rolling it on a board.        
We want the core overweight prior to swaging. Excess is bled off under pressure, giving
uniform height, weight, and diameter. Calculations are as follows. In this case I’m aiming
for a 67 grain bullet:

Cut core weight = (total bullet weight) – (jacket weight) + 2 grains
My jackets average 22.40 +/- 0.01

Cut core weight = (67.0) – (22.40) + (2.0) = 46.60 grains

The stop is adjusted until they land around 46.60. Plus/minus variations of 0.10 are
acceptable.
The core cutter - I almost ordered this until my dad said to hold off. After rummaging
through his shop he found one he built in the early 1980s. Like many of his contraptions it
wasn’t pretty but was way over-engineered. The plates are half-inch steel tensioned by a
small-block Chevy valve spring. Its cutting inserts are machined out of 4140 and honed
0.192”. This supports the rod and still gives it adequate room to drop.