As noted by dick and my first post, the number of strain cycles to a fatigue failure (strain embrittlement) would be in the millions, most likely many millions, for a shotgun barrel during sevice use. If this were a real issue, we would have documented examples and guns would be equipped with firing cycle counters.

Imagine the number of strain cycles suspension bridge cables go through - yet, they don't fll down from cable embrittlement.