The Orphanage's Stu Maschwitz has created a new Macro for Fusion that makes creating muzzle flashes and subsequent interactive light flashes on anything nearby extremely easy. (And fun!)
It basically uses the input from two Paint tools, and relies on the idea that the muzzle flash matte and interactive light matte can be simple paint strokes on black. As a result this macro can be quite fun. When viewing the muzzle flash macro's output while painting the flash matte your simple brush strokes become glowing fire as you paint!
The Macros section at www.eyeonline.com (http://www.eyeonline.com/) outlines the tools function and application. Nothing too difficult of course!
The Macro is Available here (http://www.eyeonline.com/Web/EyeonWeb/Techniques/macros/macros.aspx)
(Please note: Since it is recommended to use this Macro in floating point color space, DFX+ users may have limited functionality.
Also, if you haven't done so already, make sure to check out the other Orphanage macros available on the eyeon site including the instant lightwrap 'Suck Less Merge' and 'Exposure Deluxe'.