Dang! no wonder my ears were burning!
I dont know if I can help, but I can tell you that I tried using a burr like the one in the photo above. No good. One of the most important things when cutting any material is getting the chips and shavings out of the hole. The gummier the material, the more important it is to keep it cleared. That roughing burr has not got enough flut depth to clear the chips, even when you flood it with coolant/oil/WD-40/kerosine (ask me how I know)
The best scenario is a spiral fluted cutter made of HSS or similar material. However for the purpose of building an aluminum mold, a D style reamer, made of soft steel will work. The problem with the D style reamer is that it has a neutral to shallow cutting edge rake that does about as much smearing as it does cutting. I find that for soft material like aluminum, you need a positive rake and a sharp edge, used with lots of cutting fluid. Here is a picture of one of mine:
It was made by cutting a cylinder on the lathe the size of the finished cavity, then blending the profile with a file until I saw the boolit design I was after.
Then I mounted it in the milling machine and cut a radiused groove in it to create a single flute. Back to the lathe and used a file to relieve all but the very cutting edge. I then mounted the blocks in the milling machine vice and plunged the cavities to a stop with lots and lots of cutting oil, retracting the tool often to aid in chip removal.
I would think that anybody could do this with a drill press, if you used a chainsaw file to cut multiple flutes so that the thing would track and create a round hole.
Another tip, (at least it worked for me) with a mold made of aluminum, you need the cavities to be about .002 larger than the finished boolit. As it happens, all of my cutters cut .002 oversize, so it was a snap! I just made the cutters the size that I wanted the finished boolits to be.
Here's a few pictures of my molds that I have tried:
Incidentally, the last one is a 50 cal muzzle loading boolit that I built paper patch cartridges for.
Everybody said it wouldn't work because it didn't have a long enough bearing area, but it ended up being one of the most successful. It too, was cut with a single flute cutter like the one in the first picture.
Hope this helps!