Core seating actually accomplishes two things – it bonds the jacket to the core, and it expands the size of the bullet to nearly its final size.
Again, a special die is used to seat the cores – a core seating die. The clean core is placed into the jacket, and the jacket / core are pushed into the die using a punch. The die is adjusted so that just the right amount of pressure is exerted to accomplish both the bonding and the expansion. Depending on the size of the core (remember, different final bullet weights are possible), the die is adjusted to achieve this pressure at the top of the press stroke.
Again, efficiency in process is important. To get the cores in the jacket, one can either manually insert each core into each jacket just before placing them into the die – or – a ‘core shaker’ can be used to drop the cores into the jackets in batches. Others before me created ‘core shakers’ using holes in plexiglass layers to form a tray for the jackets, and through which to drop the jackets heavy end first, and then drop the cores into the jackets. Using my 3D printer (and with permission & encouragement from one of the recent builders of a ‘core shaker’ tray), I designed and printed my own device. This ‘core shaker’ saves a tremendous amount of time over manually placing each core into a jacket. I did a time study and found that I was more than twice as fast using the ‘core shaker’ than I was using the manual method.
Here’s a video demonstrating how the core shaker works:
Once the cores are in the jackets, they’re simply run through the adjusted die. As with the core swaging process, the auto-ejection system and the catch tray add additional efficiency to the process. The auto-ejection system pushes the jacket/core out of the die, and they fall into the tray (rather than onto the floor).
After core seating, the jacket/core combination should be .2235″ in diameter. This is just under the final desired size of .224″ – just right for point forming.