Well.. if you remember my post about the "B.L. Tech sound simulator" here
Then this may be of interest to you.
Basically this company called "Incentive Designs" have been around for a little, seems they got some projects out already, and some that are a WIP.
One of them being another sound simulator called "SMG RATTLE."
Currently they have something called the "MFG Venom" which acts as a muzzle flash. Has 4 LED's in the unit and each time a bb passes out, it flashes, which is sweet. Also for GBBR's it shines the light onto the gas, which shows gas spewing out.
I'm assume the light that it emits will work well as a tracer aswell, so a two in one kind of thing. Granted this thing is highly awesome, it'll give away your position in no time.
Priced @ 79.99 EU w/ 19.99 EU shipping.
Video 3 w/ SMG Rattle:
Video 4 w/ SMG Rattle: