Well, I flunked law school, but I did stay at a Holiday Inn Express last week...
In general, a rifle (or firearm MADE from a rifle) must have an overall length of at least 26 inches, AND a barrel length of at least 16 inches. The shape of the gun is not specified.
That is Federal law (1934 National Firearms Act) In re: Your STATE laws, no idea, have no idea who you are, this conversation never took place, and 3 folks will swear I was playing poker with them.
From a PRACTICAL standpoint, guns like that look cooler than they are practical. Why not replace the stock?