As I understood from a while back, certain AK-47 parts kits had barrels of less than 16 inches in total length. Yes, the kits are totally legal to own.
Thompson Center Arms also have short barrels that can fit a carbine, but can not legally be installed. If I owned a G2 carbine and a separate 10 inch barrel that fits the carbine, I would not be in any violation of any law.
A court case fought by TC Arms a while back was resolved in favor of TC Arms. They sold combination kits that had a carbine registered receiver and a few various length barrels including one less than 16 inches.
If the shotgun barrel is being modified with a muzzle comp/flash hider and shortened to make a new minimum length of 18 inches, the barrel would have to be shorter than 18 inches until the FH/comp is welded on.
Constructive possession allows for actual intention. Some items like drop in auto sears have no other function so they have been regulated out of the same category as we are discussing. Constructive possession can also be used to charge someone with trying to suppress firearm noise ( YES even without a real noise suppressor). If you add a muzzle extension to a barrel and tell an ATF agent that you are trying to suppress noise with the muzzle extension, you could be charged with constructive possession of an improvised noise suppressor. Intent carries alot of weight in any case formed against someone.