Worked on a 1917 Enfield receiver, that thing is HARD--tore up a HSS endmill on the first pass to cut off the "ears." Managed to tear up a carbide endmill cutting the receiver bridge to a square contour. Then heated the bridge to about 600 F; this softened it enough that it would cut with an M42 end mill and I could complete the contour work. Next the drilling-- I burned up a HSS # 3 center drill on the third hole, then a cobalt drill while opening up the fourth hole, finished the fourth hole with a solid carbide drill.
How to tap this monster? "Spot annealing" might be fine for case hardened receivers, but I did not want to chance it on "hard clear through." I dipped into my tiny supply of original "Tap-Magic", from 20-30 years ago. Tapped all four holes with the same carbon steel tap, easily, keeping the tap just barely wet. Of course, I had indexed all hole locations on my milling machine when drilling, and held the tap in a drill chuck which I turned with my fingers. Keep it wet until you get it out or it might get stuck. Not much luck with new Tap Magic.
How to tap this monster? "Spot annealing" might be fine for case hardened receivers, but I did not want to chance it on "hard clear through." I dipped into my tiny supply of original "Tap-Magic", from 20-30 years ago. Tapped all four holes with the same carbon steel tap, easily, keeping the tap just barely wet. Of course, I had indexed all hole locations on my milling machine when drilling, and held the tap in a drill chuck which I turned with my fingers. Keep it wet until you get it out or it might get stuck. Not much luck with new Tap Magic.