to make the flash illuminate the turret you must make the flash a 3d object (I'm assuming that is how you did it above, sorry no photoshopping today!
) Then make sure that the objects luminosity is set really high 95%-100% sounds good. Then open up light properties, click Global Illumination and turn radiosity on. That should do it, make sure that your AA is on Enhanced Low or else it will take ages to render. Should only take 5-10 minutes for this shot to render. Good Luck