I've made a bunch of firing pins using O1, usually drill rod is closest in size, can be had metric, fractional, letter and number sizes. For flat pins, I've used ground flat stock in O1. Torch hardened on a plate with anti-scale, quenched in oil, tempered in a toaster oven. If you have to have grooves of some sort, make sure you've radiused the corners, you'll have outstanding stress risers otherwise and will be soon making another. I generally polish everything on a firing pin before hardening. Nicks and notches lead to trouble.

Just as a point of information, did the old pin fail at the groove?

Stan S.