I'm not a machinist, but I do have 2 milling machines, a lathe, and a surface grinder in my garage.
The tip of an end mill will produce the square corner. Does the nature of the machining operation prevent you from cutting it that way?
Need to do a shaper type of cut? Grind a tool, lock the spindle or put it in back gear, use an down motion with the quill to take .002” per stroke.
I would locate those two corners and drill to depth with a 1/16” drill, machine bulk with a 1/4” endmill and clean the corners with a 1/8” endmill. Then a sharp chisel if further cleaning was needed. I’ve used the single point broach method on my endmill as well but you’ll have to make the tool for it. Not really hard to do, just need to get your clearances right. Good luck!Dave
If the bolt probe was the same diameter as the pellet, it might prevent the magazine from turning on the pellet.