The pistol argument would have a little more teeth if it had a factory rifled barrel. Taurus tried to come out with a 28 ga revolver and last I heard the feds stopped them dead in the water. Not only do barrel length and overall length apply, but the gauge part also makes it a shotgun. Any time I do anything I write the local ATF. They will get back to you eventually. If they have a problem with something you have, then you have it in writing that they approved it. May take it away but at least they wonat take you away. You really can't very well say, well I read it on the internet and other folks were doing it or said it was legal.
That's like the contender. For years on here folks would claim it was legal to switch out a pistol barrel to a rifle receiver and back. It's only legal to switch a rifle barrel onto a pistol receiver, then you can't legally go back to a pistol barrel on it. But 99% of the people would tell you that you could switch them at will because that's how the gun was designed or NFA rules didn't apply to a single shot.
Bottom line you chop a barrel down under 18" without a stamp on something with a gauge behind a number, without a tax stamp that's an invitation to trouble if you don't have a letter from your local ATF saying it's okay.