I've never done that. Just sayn'...
I've seen that callout before, and I suspect that you'll find that those numbers, or at least some portion of them, show up within the same bolt. I've never measured it in any way (I learn this from the mechanical/repair trade, removing broken ones, breaking unbroken ones, making tools and parts out of said bolts...), but I do know that the hardness varies throughout any "graded" bolt. Hardware grade anyhow. Like I said, some specialized or proprietary ones can throw you a curveball, but 90 percent or more of all of them you ever see will fit the mold. I think it's a mechanical hardening from the forming processes, drawing, head stamping, thread rolling, etc, because the areas that I believe get "worked" more during forming, that's where they seem to be the hardest. But yeah, hardness is relative, and without even trying, you'll find lots of materials "harder" than grade 8 bolts.
It might not apply to this project, (or maybe it might), but even the small allen bolts, (socket head cap screws...) the ubiquitous black coated ones (and almost all of them), which are notoriously as hard as a coffin nail, those actually fall in a quite similar hardness range, but something like 20 percent higher tensile strength. You can drill and cut those with high speed steel too, when you're making special fasteners to make your projects fancy.
