If you dont mind a suggestion. If your drill pess table will spin upside down, put a 90 degree block up and shim the receiver out. This will put the buffer ring flat on the table and square. It will be most stable this way. When you drill youll be going though the center hole of the table from the back side and the receiver will be true. You just cant press hard, let the bit cut. Its hard to discribe but i did it at a friends with great results. I can set it up and send pics if needed.
BTW how did you do such nice work on the fcg pocket without a mill??