I have to second what dano said - it was the first thing that popped into my mind, which is why the other thing (a bullet while peeping through one of those), probably won't be the second thing popping into my mind. Something tells me that the best way to see what's behind a door (for now), is a flashbang, a big ram, and a 7 man team weaving through the entrance and sweeping every possible corner/elevation within that room.....but that's just my take on the issue. One day they'll have devices for seeing right through the walls that are inexpensive enough for you and me to experiment with, but for now, that peephole thing is as good as painting a target over your eye......unless you just want to use it for playing pranks on friends who won't shoot back.