This is to put in the left over space after you have extended the barrel. To install, you will need to start by removing the six screws in the retaliator barrel extension. There is a rim in the middle of the barrel. Move the barrel out so that this rim sits against the orange end of the barrel. You will also need to sand off the inside edge to make the orange parts reassemble the barrel properly. You may want to sand the inside of the barrel to prevent any misfires with your retaliator.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/bD0puhBuxaP