If the barrel is not on the rifle, it is just a barrel.
However possessing a short barrel and a gun that it could go on with no NFA paperwork is constructive possession (illegal).
While I think you could do what you are talking about legally, it seems you are treading on dangerous ground.
I would just buy a upper that has the 14.5" barrel and hider pinned on and sell the old 16" upper you are looking to cut.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer