How does S7 tool steel compare to the milspec 8640/8740 steel used for firing pins?
I haven’t seen lysander here in a while. 
The reason I’m curious is most AR9’s use S7 or 17-4 but they break.
So I wonder why they don’t use the same 8640/8740 that AR15 firing pins are made of?
Why S-7?
- more common (less expensive)
- easier to heat treat
- suitable properties for a firing pin
- “tool steel” sounds better than just “steel”
Why 17-7?
- corrosion resistant
- adequate properties
- easier (cheaper) to get than 8640
- “stainless steel” sounds better than just “steel”
They really shouldn’t be breaking, I suspect it’s a quality thing, not a material thing.
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I don’t think FP breakage on quality AR 9mms are a common problem. I’ve got 10s of thousands of rounds on them, and train with 9mm specifically because they are tolerant of high mileage with low maintenance and inexpensive garbage ammo.
If you follow the some of the AR9 forums and groups, there are quite a few posts about broken firing pins.
Would 8640/8740 be less susceptible to breaking?
After all it is the tried and true standard for the AR15/M4.
The M16/M4 uses 8640/8740, but I have seen 2024 Al, 1060, 1070, 1080, 41XX, 43XX, 3310, 9310/9315.
The reason the M16 used 8640/8740, was probably because the Ordnance Department liked those steels (with varying amounts of carbon), and used it in the M1, M1 Carbine, and the M14. Others prefer other alloys, H&K seems to like 9315, most machine guns use 4340.
There may be another reason for firing pin breakage - poor firing pin design.
The FAL had a firing pin breakage problem all throughout its testing in the US, until it was finally resolved in 1955-56. The problem was the FAL firing pin is at an angle to the bore axis, about 11 degrees, and the hammer hits the firing pin normal to the bore axis. This means the rear of the firing pin has two force components, a forward component and a vertical component. The firing pin was constrained in the vertical direction by the retaining pin, so the hammer blow put a bending moment on the firing pin. The bending moment was sufficiently large to subject the firing pin to low cycle fatigue and break.
The solution was to leave the firing in two pieces. Since it broke at the point of maximum moment, leaving it broken eliminated than moment and reduced all the others. The retaining pin was relocated rearward to retain the back half of the firing pin.
The point I am trying to make is this:
If you are breaking a S-7 tool steel firing pin, something is wrong, and the fact that it is made from S-7 is not the problem, might be poor processing, poor design, poor execution of the design . . .
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Thanks for the explanation and details.
I did reach out to two manufacturers and both sited that the problem was the harshness of the blowback design.
I couldn’t help thinking there’s more to it though…