I found a product called Grancrete, that when laid down with a trowel and embedded with multiple layers of fiberglass can be made bullet proof.
I was originally investigating this stuff because it was invented for nuclear waste encasement I thought it could be used for a little extra protection. After talking with a very knowledgeable staff I found out that it has never actually been used for nuclear encasement but they have completed ballistic tests and it was resistant up to a .30-06 with just a two-inch layer.
I do believe that people could easily retrofit a house [with Grancrete] to add protection. Maybe not to protect the entire house but at least a few feet strip around windows and doors that could be done aesthetically. most people don’t realize how easy it is to to get shot through the wall.
I also found a much easier way to make sandbag walls at a greenhouse building web site. Here they sell rolls/ tubes of sand bags. they make an easy fill-in-place solution and and direction on how to build with them. Thank You – Danny