This video describes how to build an interior bulletproof shield to keep you much safer in your safe room. To avoid any potential mold problems, make sure the stones you put in the barrier are completely dry. I used Vigoro Pea Pebbles. They come wet in the plastic bags so I dumped them out on my garage floor, raked them out and let them dry overnight.
PLEASE NOTE: Contrary to what I say in the video, the corner posts are actually 2X4s instead of 4X4s. Also the plywood is strong, good quality 1/2 plywood. In some replies I noticed I incorrectly said 3/4 inch plywood. If you like you can go to 3/4 inch plywood. I have actually leaned the entire wall fully loaded with stones out from the wall at about a 20+ degree angle and it is solid without and bulging or any other issues. However, you can make it as strong as you see fit.
PLEASE NOTE: This wall adds a lot of weight to any floor where you install it. Make sure the floor can take the weight especially if there is a floor below the one that holds up the bullet proof wall. Even if your house is on a slab, make sure the area you place the wall on is good and strong enough to hold it up without damage now or in the future.