[ThunderCactus]

65 degree hardness what
Because if it's 65 rockwell B, it's pretty much as soft as steel gets. Leaded steel.
If it's 65 rockwell C, it'll shatter the first time the bolt hits it.
Vickers HV or Brinell HB, it might as well be a week old cheese.

Problem with hardening is you first need to know what kind of steel it is in the first place. Given that it's even actually steel to begin with.
Your standard 1018 low carbon steel comes in at 71 rockwell B. So this is either 10L18 (leaded) or something with even less carbon in it.
As you can imagine, leaded steels are gonna be garbage for hardening.
If you luck out and it's actually a carbon steel, and actually something that can be hardened, then the top end it going to be fairly low.
From back when I used to do it, I believe the top end of 1018 was about 110 rockwell B (quenching from 1550F into cold water), but for an impact component you'd want to temper it down to something between 90-95 rockwell B.
Hardest thing about heat treating a component, you have no freaking clue what hardness you're actually ending up with without testing.

What's really fucking hilarious, is that if it really IS rockwell B scale, then your bolt catch would be better off being made of 7075 aluminum, since it comes in at 87 rockwell B with the right temper. 6061, the traditional "aircraft aluminum", is around 60 rockwell B. So there it is, you might as well have an aluminum bolt catch lol
Is it magnetic? Or are they a bunch of liars?

Good idea, though. Probably worth trying if you can confirm it's a carbon steel.