The CFC has no record of a barrel I just bought, so they have no idea how long it was in the first place, I have a registered lower receiver, and no where on my reg cert does it specify my barrel length, I can shorten my barrel to 7 inches if I want,and even if you reg cert says you have a 16in barrel,you can always buy a shorter barrel and re-register the rifle with the CFC, no different here, except I made the barrel instaed of buying it.
For example, I have a 16in barrel in front of me, ooops I just cut it, the CFC never had a record of the barrels previous length, so who's the say it wasn't 7 inches when I bought it?