It is always necessary to know the distance factor. I am sure even a smallish book could stop a 8 mm projectile at sufficient distance or after a ricochet. BUT he would have had to hold a number of phone books to stop a 50 AE at less than a meter. I built a range for a forensics lab once, using phone books. No problem stopping 7.62x39 at 1 m in one phone book BUT we used to layer the books end on so they didn't shred too quickly.
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