shortgrass
Silver $$ Contributor
Steel that is already spring tempered would be quite hard to machine. Especially a part a slim as a firing pin. Spring temper is 50C to 53C. The firing pin could be made of high carbon steel (1070 to 1095), and then hardened and drawn back to 'spring'. What is needed for a firing pin is shock resistance. Spring may not be the best for this. Heat treating and then tempering for 'spring' is quite critical. S5 or S7 are good 'high schock' tool steels. But, I'd suspect that good ol' 4140HT would work just fine, or maybe 4150 which can be hardened and drawn to a bit higher degree of 'hardness'.