It becomes a rifle barrel when it's capable of being used on a rifle. Chambered/threaded whatever.
It's not the barrel that's the problem. You can have 1000, 8 inch barrels with no trouble. They're unregulated.
The problem is when you also have a receiver/frame that the barrel can be mounted on.
THAT is "constructive possession" of a controlled item.
It actually seems that the ATF is letting up on the really ridiculous interpretations, but if you've got a short barrel and no handgun for it to go on but you DO have a rifle it could go on, you'd be in serious trouble if ATF found out.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.