One of the most affordable lights that both first responders and civilians carry is the Maglite.  If you carry a full size flashlight, like the Maglight (rechargeable or standard), the cap alone will not break glass (see our videos).

There is no way you can break a window with these flashlights without causing damage to your expensive light or injuring someone from flying glass.  Attaching Bust A Cap® to your flashlight will insure that it will break glass with just one tap and not damage your equipment. 

Warning: Please remove the o-rings from the old cap and put on your new Bust a cap to insure that your light remains water tight.