While making grains for ex motors, I found that the larger the motor the more waste there is when you drill the core. Drilling a core on smaller motors say like 29mm and 38mm are ok because the waste is small and you can use the cored propellant for other uses like making igniter's. But with bigger motors it is much more economical to use a mandrel and a coring base.
The coring base is a item that allows you to install a casting tube into it and has a centered hole for a mandrel. I have made these coring bases out of wood and aluminum.
Mandrels can can be any round rod such as a wool dowel, Derlin plastic or even pvc pipe. Your coring base should have a good snug fit so as not to allow your propellant to leak out of the bottom.
Your mandrel will need to be coated with something to allow its release from the grains(except Delin). I use wax paper and packing tape for this purpose. First a small piece of tape to hold the wax paper one. You only need to wrap your mandrel once with wax paper and it only needs to be as long as the mandrel. Once you wrap the mandrel with wax paper, hold it tight and used clear packing tape to wrap around all the wax paper. The packing tape with stick to the grain when it is cured and the with a slight push on the mandrel the tape holding the wax paper will let go and you can easily slide the mandrel out of the grain. The nice thing this is a great way to store your grains until you are ready to use them. Then just use a small screw driver and needle nose pliers to remove the tape by first sliding the screwdriver carefully between the propellant and the tape on two sides then using the pliers to twist the tape out. It takes a little practice but it works very well.
Photo's of the Coring Bases and Mandrels