As I understand it, you fill out a form 4, like you would a class 3 weapon, go through the background check, get your form signed off on by the local constabulary, send the money for the tax stamp, and you must dedicate a lower receiver that is marked, SBR or Short Barreled Rifle. Now if you have a lower receiver that you purchased to make a pistol out of, and stated that when you filled out your 4473, the complete upper with the short barrel could be ok, to just purchase, but if you don't have such a receiver, and you do have a rifle lower or complete AR rifle, you could be in trouble. As you have the components to make a short rifle, and no other reason you can prove. The pistol loophole is what allows you to buy one without a form 4. Some people will buy the short upper and permanently attach a flash hider that brings it to 16+ inches, this never turned me on, but different strokes for different folks.