The 'stem' is a piece of .308 Winchester or maybe a .300 short mag. case glued into the wood. The hole is the primer flash hole. I chuck them up on the lathe and turn them back to the bottom of the primer cup, then cup them with a drill bit. I used to do this with files after chucking them in an electric drill, clamped in the bench vise. That was my 'lathe' for many years.

Note the longer shanks on my starters. these put the ball a good 4" to 7" down into the bore. the farther down they are, the less chance of breaking the rifle's rod when loading. I know a lot of people use muzzle protectors, but I never have, in 38 years of shooting muzzleloaders.

BTW - the wooden starter's knob, is a piece of smaller diameter base-ball bat. Come to think of it, it's probably oak, not kickory.

I also use adjustable measures when working up a load. Once I find the most accurate load for the gun, I make a brass, bone or antler measure to throw that amount. Over the years, I've made many measures and pretty much have them 5 gr. increments from about 25 to 120gr., plus my 140, 165 and 200gr, measures.