If you do not have access to a mill, using a router based jig is your best bet. knfmn is absolutely correct, drill presses were not designed to, nor do they like to, cut latterly. They are designed strictly for vertical cutting. Even if you do manage to get though it, it will not be pretty.
Using a router based jig, like this http://www.northeastshooters.com/vb...s-AR-15-Jig-plus-80-Lower-Receiver-for-199-99
Will give you the results of a mill at a fraction of the cost.
Using a router based jig, like this http://www.northeastshooters.com/vb...s-AR-15-Jig-plus-80-Lower-Receiver-for-199-99
Will give you the results of a mill at a fraction of the cost.