Regardless of the care taken in annealing, likely brittleness would come on quenching, as the original thought was to create colors/hardness?
Issues of hardness vs brittleness are taken care of by tempering.
But, could a thicker case result in more, for example, tang cracks at screw holes, or warping due to the higher stress an increased percentage of high carbon steel can put on a part when it's hardened?
See above.
A curiousity is that gun part annealing, thought to be critically important, is regularly said to be done in a charcoal pack.
A charcoal pack will easily reach annealing temperature , even for high carbon steels. Hardening temperature is usually a bit higher than annealing. Ergo, the part is annealed prior reaching hardening temperature more or less automatically.
DDA
