Might make for some interesting reinactments. They have these neat 'tracer' rounds that glow extremely bright (they also have magazines with built in UV lights to 'charge' them up) that would look a hell of alot like lasers...
But yeah maybe you could make a nice mix of black powder with barium to get the green glow (although it should be red I dunno what stuff you put in, maybe phosphor), then put the mixture in a hollow wooden or metal casing (sealed at the back), with a touch explosive , something ammonia-iodine based perhaps, so it will ignite when you fire it away