This is the 3rd mode you will get for any gun model. This model focuses entirely on the assembly of the gun after it has been fully disassembled. Unlike the Disassembly mode, the pieces will not be highlighted this time around. This is a good point to start testing your memory skills and click the pieces in essentially the reverse order of what you did for disassembly. Once again, if you have any issues finding that right piece, you can either click "H" or click on Hint on the top right of the screen and it will take to the next piece.
Note: There are no penalties or limits to using the hint option in this game mode.