I built a rig for doing this using LED strip and a microcontroller board, so it could be timed to sync with the shutter and avoid flash banding on rolling-shutter cameras, as well as ensuring that the flash was actually visible. The actual light-emitting device was a load of LED strip stuck on a bit of plastic, equivalent to about 20W of LED, which wasn't really bright enough - I've since bought a much more powerful emitter and controller board for it but I don't currently have much use for it, so it's a backburnered project right now.
There were two ways of triggering it. Some of our prop guns had electronics in them to trigger the flash via a TV-remote-style infrared communications protocol, and I built a tiny PCB with a microphone to allow things to be triggered from the soft "pop" produced by an airsoft gun, where there wasn't room to include electronics.
In bright lighting conditions it doesn't make much difference, but in the dark, I think it's quite effective. This is ungraded, without the flame effect composited in.
In this case it's infra-red triggered and the emitters are hidden in the mock sight on top of the machine pistol he's holding. Batteries inside, power switched on and off by a reed switch actuated by a magnet on the removable magazine. The cheap trick here is that it's very backlit so it looks like he's being revealed by the muzzle flash.