Don,
The casehardening I had these experiences with were not guns, but they were casehardened articles in the nuclear/aerospace biz. I was a toolroom machinist modifying these parts. I don't know exactly what process by which they received their casehardening. It could have been any of the "casehardening" processes to a low carbon steel.
I've milled and turned such parts over the years. The basic approach was always to take a deep enough initial cut to get under the hardened layer. What resulted in all circumstances was the typical dull milled/turned finish of the softer metal and a transition to a shiney-er (technical term) surface finish in the harder areas. It's common for machinists to get a "feel" for how hard metals are when they machine them. This comes from not only how hard they percieve the cutter and machine are working, but also what the finish of the metal looks like. Harder steels leave a much finer finsh. The harder the steel is, the finer the machined finish. Conversely, soft steel leaves a dull and sometimes rough finish (depending on grain size). Maybe Miller can cite some of his experiences as well.
