I was in the barber shop one day and was reading a technology magazine while waiting. It had an article about a cannon that is already being tested by the military that sends out sonic waves which force some kind of reaction within the bodies cells making them agitate. This agitation causes the person or persons it is pointed at to quickly get a burning sensation, as though they were on fire. While highly painful it causes no long term damage other than possibly psychological. The article stated that the area it was effective in could be made as small as 1 inch by 1 inch or as large as 25 feet.

Basically it's a non-lethal cone of imaginary fire.