A long time ago in a good shop working night shifts with too much time on my hands I made several fn tools.
Measure the oal of a 22 cartridge, then subtract what you want for your fn. Cut a piece of 3/4" square stock to length. Drill the diameter you want the flat to be then turn it over and drill the body width to where you have just the amount you want removed sticking out the top when placed on a flat surface. Heat to a dull orange/light cherry and tip into a 16 oz beverage can with the top cut of and 2/3 full of motor oil. Doing three in a row will cause a boil over of stinking oil that will take a while to get clean. I saw no improvement in groups but a big improvement in one shot kills on snowshoes.