use stress proof or fatigue proof steel
no heat treat required
drill rod ( o1 or a2 etc) is too brittle and will fail over time



I prefer to set up in lathe (made a 5c collet with clearance for leg of firing pin) and face back around 1/8 inch then drill hole bigger than firing pin and make press fit pin
with shoulder same diameter as old firing pin and then smallest diameter to go through breach face ... simpler than it sounds and very strong.