The tax mentioned is a warning that because it is short barreled and has a shoulder stock, it needs an application and tax to be legal under federal law. Dump the stock and it is a pistol and legal.
I buy my firearms from FFL dealers as I’m not handy at building things, simple home repairs are a challenge for me.
If you look about three inches below the shooters left elbow, in the picture, you will see a piece of ejected brass falling down.