When it comes to legal matters you have to think about what the law actually says, and how a court of law is likely going to interpret that law relative to your situation.
So imagine you own a shotgun and a short piece of pipe that just happens to fit onto the shotgun where the barrel would be installed. It doesn't do any good to own these unless you're going to use them, so one day you drive far into the woods where there's nobody around, you install the short barrel and you fire a few shots into the side of a hill for fun. Then you disassemble the gun and head home. You're pulled over for a minor traffic violation and the cop sees the gun sitting in the back seat and uses it as probable cause to search your car. he finds the shotgun and the short barrel and arrests you for possession of an illegal firearm.
Pretty soon you're standing in a federal court, a jury is about to decide whether you are goign to walk free or be tagged as a felon and spend a couple years in federal prison with a cellmate named "Bubba the sodomizer". The federal prosecutor argues that owning the barrel and the shotgun shows clear intent to violate federal firearm law. he also points out that the forensic investigation found gunpowder and lead residue on the inside of the illegal short barrel , thus proving that you had recently installed and used the barrel and committed a felony in doing so. He also points out to the jury that by removing the barrel before transporting it back home you were showing that you were well aware of the law and the serious nature of your criminal actions and were hoping to create a legal technicality by separating the barrel from the gun - because if you were just an ignorant dude who took a hacksaw to his 870 without realizing that SBS's were illegal you wouldn't have thought about removing the barrel for transport.
Now, how confident are you in your legal technicality with your life and the integrity of your rear end hanging in the balance? Personally I'd say the odds are against you, but if you want to take that chance instead of just getting the federal tax stamp then you go right ahead. Its not my job to stop you from ruining your life over a silly gun.