Neil,
Does this action have a set trigger? Often times if a set trigger was installed and the hammer didn't have a fly or slot for a fly, an easy fix was to file off the half-cock notch - if it was used for shooting matches, it wasn't a big deal - breech open or ready to fire.
Greg

I haven’t done such a repair (yet; I have a Stevens 44-1/2 hammer that I’m working up the nerve to redo). But it seems to me that most of the heat you will be applying in order to rebuild the half-cock notch will also flow to the full-cock notch, and you may even wind up getting metal in there that you will have to file off and recut that one too.

Heat-stop paste would certainly stop the flow of heat to the front, top and toe of the hammer, but you are going to wind up needing to case-harden the new notch(es) you cut, which typically means another round of heating the same area for a longer time than the most precise welding, to get the carbon into the metal for hardening the notches.  This might be too much for the heat-stop paste to handle.

If you have color hardening you want to preserve, I guess it’s worth a try, but given the fact that you will have to harden the notches anyway, I would think that annealing the whole part and rehardening it all at once afterwards would be the easiest way to go.  You could even get it pack-hardened in colors by one of the heat-treating outfits like the one singleshotsam uses.

Alternative solution: get a new one from MVA. $90, what's your time and risk of ending up with a bodgered up hammer worth to you?

If this works here's a pic of before.

I took a copper sheet and placed the edge where I wanted the bottom of the notch to be and basically splashed the weld over the copper and towards the bottom of the hammer. The copper should have helped with the heat and weld won't stick to it. My ready made safety notch waiting for some serious filing. I had painters tape protecting the full cock sear covered in cool tool and the tape wasn't even scorched.

It's a win! I only had to put it in and take it out twice. After the bulk was removed with my dreaded Dremel tool (dremel must be an abbreviation for something that messes up otherwise good parts fast) I filed out the rest. My first try (in the pic) had the notch to angled and the sear would slip. I squared the notch and it works now. I still need to polish it a bit and I'll watch for wear. If it wears I'll harden it with kasenit.

bobw, these rifles do seem to like having a very square notch. I just took off a little of the angle and it holds now.