If you have a local heat treat company, you can probably get the steel treated for a reasonable price if you're nice. Most any steel scraps you find should be able to harden enough for target use but ideally you want to know the AISI or equivalent alloy. You can also do amateur heat treatment with tons of information found online (air, water, oil, etc) but you would have to bring it to a machine shop or similar to verify on a hardness tester.
I believe rifle-rated plates are hardened to 500 Brinell, which is about 250ksi or 51Rc. That would seem to make sense since anything pushing over high 50's in the Rc scale would be brittle and chip/crack easily (ie most knives are 58-62Rc).
All the different hardness names are just various types of scales, like Farenheit, Rankine, Celsius, and Kelvin. You can convert between them using exacting constants and formulas.
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