I would say you'd have to go with an AR500 plate steel to stop 5.56. 9mm can be stopped by mild steel, but it still would need to be pretty thick (1/4 inch?) and adds a lot of weight. I've never seen the weight difference between 1/8" mild steel vs. 1/8" AR500. I can tell you that 1/4" AR500 is used in the lower priced armor plates, and is NIJ rated up to 7.62x51 multi-hit. A 10x12" plate weighs 7.5lbs with the spawl shield coating, so I'm not sure what the actual weight is. I'd think a 4x8 sheet is probably 300ish pounds.
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