First off, have you checked to make sure that it is legal in the State that you live in? Some States have restrictions tighter then Federal restrictions ( of course, if your State does allow it, you still must get the BATF approval ).
Another thing to consider, is politics, unfortunately. If you can do it, legally, and go ahead with it, think about what might happen IF you ever need to use it...if you live in an anti-gun type area, the fact that it has been shortened so much could persuade a Prosecuting Attorney to charge you, or a Plaintiffs attorney could use it against you in a lawsuit. Like it or not, those are simply facts that you have to consider. Personally, I'd use a factory made 18" pump, or 18" double; I can't think a few inch shorter barrel would really be worth the potential headaches. Then again, that is the same reason, when asked, that I don't suggest using a "military looking" rifle for home defense, like the AR's, AK's, etc. An M-1 carbine, Marlin Camp, Ruger thingie ( forget what it was called ), lever action, etc. etc. would work just fine, and present a much lower profile when viewed by anti-guns folks. Not that I like the idea of having to consider such things, but like I said earlier, it's just a fact of life.
Whatever your choice, good luck.
Mark