My 2 cents…probably all it’s worth. 
Original firing pins were most likely made out of case hardened 1010/1018 or 1020 low carbon steel.   
The socket head cap screws are normally made out of 4140 steel (if they are made in the US, if made in China …possibly bubble gum).  Nothing special about them. 
If I were making it I would use 8620 and case harden or pre hard 4140. 
Either can be gotten from Enco 
Just my thoughts

V/R

Mike

Hunter Restorations
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wesg

Yup my bad, just looked up the industry standard for Grade 8 Hardness, Rc 33-39.   

And yes a lot easier to source

V/R

Mike

1144 is not very hard at C17 compared to 4140 at C32. It does machine very well. If you get the stress proof I believe it is a little tougher.  I used a lot of it when making pins for heavy equipment. Not sure if it would hold up to well for firing pin without case harden.