Maybe I oversimplify stuff, but I tend to use the tools I have. To countersink a machine screw, I drill the hole in the metal part one drill size( not number drill ) larger than the shank, take the head of the screw, or bevel headed machine screw, and stick in in my drill index until I find a size just a little smaller than the head. Running the drill, or drill press slowly, I stop the process just short of the shoulder of the bit. I then get out my trusty old hand powered countersink, and give it a couple of turns, and it’s done. Is it perfect? Nope. Is it as good as 98% of the old time gunsmiths? Yep.
Hungry Horse
