This is the second game mode that you will do with any gun and focuses only on the disassembly of the gun. The game mode will highlight pieces that you can take apart from the gun model. All you need to do is select the highlighted pieces and then click the green arrow that appears to take apart the piece. Sometimes it can be very hard to see the pieces if you are learning a new model, so press "H" or click Hint on the top right if you need and it will take you to the exact piece that can be taken apart.
Note: There are no penalties or limits to using the hint option in this game mode.