You have to be extremely careful rehardening a reciever, because warping can happen. You need to be prepared to mill/sand it a bit to get the tolerances right afterward. It also depends on how much heat it takes when the stock burns off. I have seen a stock burnt off on a gun that was sitting close to a fire. It took literally hours and hours sitting there (it's owner stupidly left it beside the campfire all night). It only took 500 degrees or, but it took it for a very long time. This tempered the steel quite a bit, but the heat levels weren't so high that they warped it. So it was extremely easy to reharden it. Most recievers are probably nick-chrome, which usually requires 1400 degrees or so, then somewhat quick cooling for hardening(do not use water!!!!), then you need to stick it in a stove at about 375 or so for 1.5-2 hours and then pull it out, bury it in sand or preferably in ashes, and let it cool off very slowly. This should get you roughly RH 55-65 depending on the metal, so you have to do some tests first, and preferably some research to verify the type of metal exactly.
For a quenching fluid use vegitable oil, and make sure you quench it evenly, not at an angle (I have a little winch that lowers my blades in and out of the oil, very uniform).
Most of my experience comes from bladesmithing, but I have rehardened a couple recievers, and they work great. Best advice would be to have a professional do it though.
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