With AR receivers, never having been assembled as a rifle or a pistol, I can see where there would be a limitation, but a Handi receiver HAS been made into a rifle. Once made into a rifle, it will always be considered a rifle and your FFL over in AL should not have any problem with transferring to you, since you can buy a rifle/shotgun even if you don't live in AL. You cannot buy a handgun in AL
Talk to him and explain the difference. He might relent.
With the AR receivers, it is all intent. When you buy one, you can put it down as a rifle, if thats what you are going to do with it, and a "pistol" if you are going to make it into a pistol. Same chunk of metal, but intent is what classifies it.