The ATF says that if a gun starts out as a long gun (which most Mossberg 500's do), that you're not supposed to put a barrel shorter than *18* inches on a shotgun unless you pay the $200 short barrel shotgun tax to convert it, which I think is stupid as long as it is in a legal configuration I don't see what it would matter.
Recently, trump has rolled back a lot of agency determinations unless there is a written law that says whatever their determination does, and of course, there isn't, but that's pretty much the jist of it.