so to be clear if i bought a complete upper say in 10.5 and i already own a 16 " AR just possession of the lets say "pistol upper" that can be considered constructive possession?
If you do not have an SBR lower or Pistol Lower to put it on, but you do have a rifle lower (the one your 16" upper is on now) then yes, you are in possesion of an unregisterd SBR as you have all of the parts to assemble one and no legal use for the short upper (the SBR or pistol lower). If you had the short upper and no lowers you would be fine.