That’s because it was printed using copper and wood filled filaments from FormFutura, whose site states that these spools contain 80% copper and 40% wood respectively.
Before printing Thingiverse user MrNebby22 tells us that the gun was modelled over the course of seven days in 123D, using only the game on a second monitor for reference.
With that done printing took around 45 hours for both parts, which required a bit of sanding and cutting to fit together. After some glue was used to make everything permanent, no more sanding or other finishing was done as the filament provides such a good look on its own.
If you’d like to make your own the files are available for free from Thingiverse. Those files may change in the future to add in the missing details of the tape on the handle and the wire around the barrel, but this could always be added with actual tape and wire you have lying around.
As it stands right now the finished version on this page is 39 centimetres long and looks just about perfect in the hand.