The door can be the most important part of any defense plan. In an active shooter situation, the goal should be to limit access to as much of the building as possible, and active shooter security devices do exactly that. By transforming a door into a basic barricade, it effectively limits where a shooter can go; after all, if he has problems getting through a door he is likely to go elsewhere.
The idea behind most of these security devices is pretty simple: They quickly modify a door so that it difficult, if not impossible, to open the door. They either go over a hinge, wedge the door shut, or otherwise prevent the door from opening, transforming the door from an entryway to a barrier between the shooter and potential targets. These simple devices can easily make the difference between life and death, making them a great investment.