I just can't see why you can't make one " Legally".
The legal danger in attempting to build one of these things yourself, is the fact they're being built and sold due to a technicality, and or new found existing loophole, in an already nonsensical gun law. These things are considered a "firearm", and not a "shotgun". So what? Now all of a sudden my 18" plus barreled shotguns are no longer considered to be "firearms"? If so, why can they be both, and this contraption can't be? Even if they fire the exact same ammunition.
When you take all of the common sense out of a law, and start basing it on ridiculous technicalities and loopholes, it doesn't legally translate well, if at all. You could wind up in court, spending tens of thousands of dollars, trying to defend yourself against something that makes little, if any legal sense.
If I were wanting one, I would simply purchase one
NEW from either Mossberg or Remington like Bubba did. That way you know what you are purchasing is legal. Even if the law that makes it legal is completely idiotic and asinine.... Which it is. But that doesn't mean the Federal government isn't going to prosecute you, if they think for a New York second that you are violating it..... And even if you are right and they're wrong. You'll most likely go broke trying to prove it. This whole cluster F gives new meaning to the phrase, "The law works in mysterious ways".