How do the big manufactors do it?  They machine out the mag well then use a broach to square the corners?  There a CNC broach that can start from a hole in the mag well?  I have watched videos on youtube of broachers run and they all seem pretty slow.  Just curious really.  I am just going to buy an 80% lower with the well EDM cut.

I'm not sure how it's "usually" done as i'm sure there might be a few different ways thats it's "usually" done. I have always thought you could mill most of the material out and use a keyway broach with a custom guide block to to fit the area milled out and get all corners as needed. I have also seen some done with small diameter long drill bits to get a radius in the corners and then mill out as much of whats left and finish up with a file.