As for comments by others about Mossberg doing their home work on this. They don't care if it becomes an NFA item tomorrow as a $1,000 SOT a year is nothing to them so they would continue to make them as NFA items.
That's an interesting point and I'm of mixed minds about that. Major American firearms makes have traditionally avoided making any NFA products for sale to the civilian market, from what I can recall. Shotguns may be slightly different in that regard as while I know it is possible to buy factory shorty barrels for some, I don't know if you've ever been able to commonly buy new factory-made SBS Mossbergs or Remingtons, new-in-box at your local SOT Class 3 dealer's shop. (Maybe so. I just know know) I think the big makers have always seen the civilian NFA market as just way too small to bother serving, leaving that to small specialty shops like Serbu.
Now, however, the mighty Ruger itself is making a silencer under its own brand, and Remington bought AAC, and well, things seem to be shifting. The NFA djini is out of the bottle it seems, and the big makers are starting to see that as a way to be cool and cutting-edge. Almost as though catering to the hardest of the hard-core gun guys gives them extra "street cred." So, while I'm sure Mossberg doesn't want this to be re-determined as an NFA item, and would be terribly surprised if it suddenly was, yeah they might just deal with that fallout, rectify the matter however BATFE told them they had to, and keep on making them for the much smaller NFA market. Or, of course, they might just drop the product. It happens. Not that big a deal.
They could simply send anyone owning on of these an 18" Mossberg barrel that cost them about $20 to make and that would make their gun legal.
That would be interesting to see because it would give some possible answers to the knotty question of, if you find yourself in possession of an illegal, unregistered NFA weapon, is it actually legal to destroy that contraband item by reconstructing it into a non-NFA weapon? The BATFE has long said that your only avenue is to forfeit such a thing, but of course they also allowed for a bit of a grace period for registering your USAS-12 when they changed its determination which might suggest some flexibility on their part.
Also, they are just as subject to an NFA determination letter as a small 07 Manufacture like me. The end user is the one that gets the shaft by risking this.
Of course, it seems that "the shaft" in this case would likely be not more than a letter saying you have to register it, for free. At least, that's how it happened before. Not a happy thought, but not like they come and throw you in prison or slap you with a $250K fine.
Also. Before anyone buys one of these they better check their state Laws. Just because it has slipped through the cracks of the NFA doesn't mean you can get away with it in your state so check first.
Reasonable advice. Now, it would be good to put together a list of states which specifically ban or further regulate an item like this, in cases where federal law does not.
One I can think of is Michigan which defines under state law that any firearm under 30" in length is a "Pistol." And so this would have to be bought and sold under their rules for a pistol. But any FFL dealer in Michigan would automatically be aware of that and would handle it appropriately.
I'm not sure what, if any, other states would prohibit these, if they're legal as GCA type "Other Firearms" under federal law. Can you think of any?