"So a "peppering" (look up the definition) of bird shot can enter thru the mulitple laters of cloth, then skin, then muscle, to lodge against the heart.
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Indeed it can. At 30 yards, it's not a problem even for #7 1/2 shot to penetrate clothing and the thin skin between human ribs. From reports, it was #7 1/2 shot.
"Peppering" is probably not the right word, really. Any shotgun wound at 30 yards is potentially very serious, especially to the face and neck. It can blind, pierce the carotid artery or jugular vein. A body shot, particularly on the side, can easily have pellets penetrate the chest cavity if it hits between ribs.
It's not a joke to be hit with even small-sized shot.
That said, this was still an accident. It was an avoidable accident, to be sure, but it was an accident. But, the dangers of being struck by shotgun pellets at 30 yards are not to be dismissed. Most often, the person hit will be OK, but not always.