You can try that trick with the Thundermaul shell overtop. It depends on the border agent you will be meeting. Since you're underaged and you're trying to get around the rules nobody here will treat you lightly, yes you will get flamed.
As for the FN2000s the custom agents are obviously don't know their guns, because I've had this brought up with the CFO and the verdict was that it does resemble a real gun so therefore its a no. Go google up the FN2000 you'll find that it is a real gun.