Opening feed rails is not a mill operation. My favorite tool for this is a 1" wide plastic handled diamond lap. I use the brass intended to feed as a gauge and open the rails with the diamond to get the release point and angle exactly where I want it. Then I use my trigger stones to polish up the edges so they don't scratch the brass. Takes me about 15-30 minutes to open up an ought six action to Ultra Mag now that I done a bunch.

"Hired Gun" Quickest and fastest all motor sand car on the planet.
3.008 at 104.8 300' of sand.

NRA Patron Life Member, Gunsmith, Instructor, Chief RSO