I would suspect the actual case hardening of a low carbon steel would make it 'stronger'. Not because the original material was heat treated, but because the case is an entirely new steel that has the ability to harden. I think the trade off with 'hardening' a gun steel is that in a softer, relatively, state, it yields slower. I think hardened steels tend to yield quickly once that point is reached.
Some, not all, of the pictures of failures look like steels that were inadvertently hardened, with little apparent yielding before letting go. But, the grain of the steel might look very course, like pebbly sand instead of smooth and consistent. It may have unintentionally been exposed to heat that didn't just contribute to generic hardening or annealing. Just thoughts.
