This is the rough Guide to creating the StAr15.
The St-Ar (Sten/Sterling/Stubbs)-(Armalite (Pattern) Rifle 15) is an open source firearm based on an etremely simplified AR15. The BATFE has many times expressed a desire to redetermine what constitutes a firearm to classify an ar15 upper reciever as a regulated part, and some companies upper recievers themselves are already regulated as firearms, making one assembled gun two separate regulated firearms each subject to tax.
After much backlash they rescinded the redefinition of the AR15 as a controlled split reciever firearm and grandfathered all existing firearms of this style.
This is not to say they will heed forever. If at any point they double back feel relieved knowing an ar15 upper will always be avalable in some form in the same manner a lower can be fabricated at home. All you need is your trusty aluminium tube and a 1.250x18 die.