Came across these while researching non-lethal rounds. Thought it would be of interest to someone looking to setup early alert system for their bug-out spot in the woods.
http://www.deltaforce.com/catalog/lesslethalammo.html
This particular device, when used outdoors, serves as an effective pest control and early warning signal against intruders on your property. This system is all aluminum construction that can be used over and over again. Designed to accommodate 12 gauge blanks. Kit contains all the necessary hardware to install where required (tree, fence, post, etc). Includes one 12 gauge blank cartridge.
Can Not Ship to California and New York
http://www.deltaforce.com/catalog/lesslethalammo.html