Thanks for reading and commenting. It is best to “hide” behind cover (solid brick or concrete walls, heavy furniture, large vehicles) if it is available but many open areas do not offer any cover. Standing up or laying down is kind of like walking versus running during a rainstorm. It was not immediately clear where the shots were coming from in Las Vegas and laying down would make your body a bigger target for someone shooting down from 32 stories above. You are also more vulnerable to be trampled by a large crowd if you are on the ground. During Virginia Tech, some students who got down on the floor were below the line of fire. It is dependent on the situation, the position of the shooters, available cover, and largely luck.