Lastly - if you read the letter ..... Mr Spencer says that to be legal, you must pay 200.00 and register the firearm with the NFA as "A weapon made from a rifle" He didn't say "short barreled rifle" as that is something else.
But I have spoken to an inspector from the NFA branch and they said that that type of classification exists, but if you did a Form 1 to make it, they would send it back to you and ask for further clarification on what you wanted to make.
If it were me, I would send it back to them with the Technology Branch letter and tell them to fight it out.
So - it is my understanding that the following is a list of all NFA weapons:
1. Machinegun ... Fully automatic
2. Silencer ... Suppressed muzzleblast
3. Destructive Device - non-sporting bore size over .500
4. Any Other Weapon - All kinds of stuff that doesn't fit elsewhere
5. Short Barreled Rifle - with a stock, bbl shorter than 16"
6. Short Barreled Shotgun - with a stock, bbl shorter than 18"
7. Weapon made from a rifle - no stock, bbl shorter than 16"
8. Weapon made from a shotgun - no stock, bbl shorter than 18"
If you go to the ATF website and look at their firearms identification pages you will see all these classifications .... there has been some debate on other forums as to if you can actually make #7 and #8.