The hollow plunger is one of the first tools needed to make the rocket propellant, so I thought I would start making this one first. The plunger is made of 1" diameter easy-to-machine steel, and turned down to 0.948" in diameter. It is 3 inches in length.
I then increased the drill bit size used to drill a hole in the center from a 1/8 inch center drill bit up to a 0.406" drill bit, in order to accommodate the center plunger.
Finished, oiled, and ready to store until the propellant is casted!