Peeteee Custom Ultimate Gen 4 Shotgun Tracer Unit
(Model: Tokyo Marui KSG)

"It does exactly what you think. It uses 3 grub screws to tighten itself to the outer barrel for nearly every Gas shotgun ive tried it on so no faults there. It charges via USB micro and will indicate a green light when done. Shines nice and bright for tracers to scare the bajeebus out of people.
But it has its downs sides. The cap that covers the charging port and on switch can come loose if you dont follow the instructions on which way to twist it on. Luckily I play at a CQB field with it so if I lose it. It managed to come back to me but I cant promise the same for you. I bored two screws into the tracer to afix some wire to keep it from being lost.
I will also admit that because of the nature of shotguns the bbs have shot the front cap off completely (evident from the plastic missing from the exit hole of the cap from the inside)
The advantage is if you do purchase the unit the "tracer engine" is its own unit so you can email the manufacture (peewee customs) and they'll give you the PDF for their entire stock with pricing for just the shells for the different style of shotguns wether it be aa12 or KSG