A few years ago there was a lot of buzz in the industry when rumors started to circulate about criminals filling their walls with sand in order to build bullet proof rooms. The theory suggests that about 3 and 1/2 inches of sand would be all that is required to stop a bullet since the standard distance between two pieces of drywall is about 3 and 1/2 inches. Sand bags have been used in military contexts for many years when building trenches and barricades so there is some general feelings that enough sand will stop a bullet.
Sand is a very heavy and dense material and so you can easily imagine it stopping a bullet but is less than 4 inches enough?
The most definitive testing seems to have been done by the pros over at The Box O' Truth in which they fired various handgun rounds and rifle rounds into boxes of sand. From all the tests “no rounds penetrated more than 6 inches” of sand. (See their article here)
Our friend over at BulletSafe also decided to put it to the test with a Desert Eagle and his results were slightly more successful but still he was unable to penetrate more than about 7 inches.
So it would seem that Sand is an effective stopper of bullets. I can understand why the military would use Sandbags for an effective cover shield. The great guys over at Tin Hat Ranch also did a balistics test with sand bags.
I'm still not convinced that pouring sand into my walls is a good idea but it would likely slow down most handgun rounds enough to at very least make them far less lethal or potentially would stop them altogether. Sand is cheap and easy to find so there are many potential applications!