I have a 9mm ar carbine. I would like to replace the 16 inch barrel with a 10.5 barrel and make an SBR. I know about the need to get the tax stamp before constructing the SBR. What i don't know is it illegal for me to purchase the short barrel before getting the stamp? Is this intent to construct a SBR even though it could also be used to construct a ar pistol which is legal? The reason I ask is because I have an opportunity to get the barrel I want at a very reasonable price right now. I have done site searches and can't seem to find an answer to my question.