I recently bought a 80% lower and jigs from you guys. It appears that your jigs are designed for use with a drill press. I have a Mill/Drill that is somewhat more rigid than most drill presses. I am wondering if I could successfully complete my lower while eliminating the first steps of drilling all the small holes using the drill bit. It seems like, due to the additional rigidity my mill affords me, I could start out by drilling the 3/8" relief holes with the drill bit  then finishing with the 3/8" milling cutter.

Also, a suggestion. When using milling cutters on a mill with a collet, there is a tendency for the end mill to pull out during the milling process (could be just a few thou or it could be 1/4" or more) I have had this happen to me on occasion. The cure for me was to buy a set of R-8 milling holders. These have a setscrew that goes into a "Weldon Flat" which is a flat spot on the shank of the end mill to keep it from pulling out. I hope you will consider ordering the end mills that your are selling with this feature.