I have a can of case hardening compound that was given to me by a millwright fellow who worked in a mill in a small community north of us. It's called Hard and Tuff, and is a moss green powder. To use, I heat the item bright red/orange in the flame of a torch, immerse the item in the powder and then hold it at that red heat for about two minutes. I quench in room temperature water. Then I dry the part, and burnish it in a fairly stiff wire wheel held in my drill press at highest speed. The result is a tang or lock bolt that is a very dark grey, polished, and hard enough to withstand a ham-fisted client who removes them on a regular basis, without any damage at all. I harden lock bridles and screws, including the wood screws for the buttplate, etc. with the same method. the hardened screws resist oxidation as well as poorly fitting screw drivers.
I ran out of Kasinite many years ago. Several years ago, I tried pack hardening using 50% hardwood charcoal, 25% bone meal charcoal, and 25% charcoal made from leather scraps. The results of that were completely satisfying. The colours were bright and varied, the steel could not be filed, and there was very little warpage.