I ran into this when I decided to build a varmint specific AR over the Christmas holidays buying on sale parts from different companies. I went to my local gun store and bought an Anderson lower for $55.00 and told them what I was doing with it. Then over two months I bought parts and built my rifle. When I went to get the serial number to add to my firearms insurance after it was completed, I saw in little letters under the ser # the word PISTOL. I thought uh-oh, and contacted Anderson who told me if it was marked PISTOL, I HAD to use it as a pistol and not a rifle. I went back to the gun store with the entire rifle thinking I had an illegal gun. I was told as long as I assembled it as a rifle and didn't go to a pistol later, I was fine. They even contacted one of the Feds that the owner deals with, and he said the same, can't turn it into a pistol now, but the word PISTOL on it doesn't matter legally.