There was a test i saw and i cant remember where but a guy took some text books and did some testing with common handgun calibers. This came after one of the school shootings and he was advocating using the books and putting a backpack on. The books stopped the rounds and i thought it was very informative. I just dont remember how many books it took. It was less than i thought. These were hardback books. It's not much but it could make a difference. I advised my kids if they are caught and cant get out to take their backpack and cover up. Again,not much but better than nothing.