It is filled with tracer BBs passing through the silencer tube and the arc and trajectory of the projectile can be seen imitating the tracer bullets used in reality.
Description of Xcortech X3300W MK2 Weapon Mounted Chronograph and XT501 Silencer Style Tracer Unit:
The control system includes a burst ECU mosfet, a tracer unit and a PEQ command unit. The number of shots between burst fire is controlled between two and ten via the PEQ command unit. 14mm CCW tracer unit with built-in chronograph that can be read via the PEQ display. The firing rate of the weapon can also be easily adjusted using the control buttons on the PEQ unit. You can set the battery magazine display for standard or high capacity magazines. The device communicates wirelessly.
The set includes:
1x Xcortech XT501 Tracer Unit
1x Attachable Control Display (show rate of fire, FPS, BB count up or down and more)
1x Battery Controller (Sets how many shots you want between 1, 2, 3, 4 or 5 shots)
Specifications Ultra Bright UV Light Tracer Unit Firing Rate Velocity Countdown Counting Total Shot Number Burst Control Display AEG Battery Level Low AEG Battery Detection (Vibration) Programmable Burst Fire Control for AEG Low Ammo Warning Tracer Unit Diameter 37mm Tracer Length 154mm
Tracer unit works with 4 AAA batteries (not included)
The command box works with 2 AA batteries (not included)