Gass relief hole botom of breech block.

[DonVoigt]

"as the pistol's metal should be the same temperature as the primers are."

For sure, but what would that fact have to do with the observation that the thin, malleable primer cup metal becomes brittle?

The "metal" in the pistol is of different composition, shape , and thickness; and would be unrelated to primer failure- or am I missing something?