Almost any plain end mill bit will work for cutting out flat openings and rails. Carbide will last longer wear wise, but snap easily if too much force is applied. A center cutting type will plunge cut which is what you need for your AR15 and steel barstock projects. I would get several in all the sizes available, up to what your machine will handle. You are about to discover that the cost of the mill is the cheap part, the tooling is a never ending expense and soon surpasses the cost of the mill. If you are on a budget, the 1/4" and 3/8" sizes are the ones I use the most. That is a good start and you can add to those as you need them.