The metal tracer unit is an awesome addition to your gun! Use at night, with any .20+ 'glow in the dark' tracer style BBs. The unit emits a UV blacklight to brightly illuminate the BBs as they fire from your gun. It makes it shoot like a laser gun. Or use mixed with regular BBs for a tracer-like effect. Also looks like a mock suppressor on your rifle.G&G has attempted once again to improve another Tokyo Marui design. The Tracer Unit from G&G has a few features Marui's version lacks like a rechargeable battery, and auto power off. Hey, it also comes in Desert Tan too!Comes with everything you need, charger, internal rechargeable battery, and installs on nearly all AEG guns with 14mm threaded barrels (CCW). Just get some tracer / glow BBs and you're all set! Highly recommended kit.