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Even a proof test won't always detect those brittle receivers; they were all proved when they were made and obviously passed.
When a normal round or even an overload is fired in them, the result is a "push" against the bolt. The receivers will stand up to that. (Think standing on a soft drink bottle - a hundred plus pounds doesn't break it.)
It is a sharp blow that shatters them. That can come from rapid gas escape from a blown case head, firing a round with a light load of pistol powder, or even a blow from a hammer.* (Think that same bottle hit with a hammer.)
*I once broke one of those receivers by hitting it on the right side rail with a light hammer; it shattered into three pieces. Yet a friend and I had fired a couple of hundred rounds from it the day before.
Jim
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