Masterkey shotgun question?

Ok we have all seen the M4/M16 style rifles with a shortened 870 mounted under the barrel.

Now I have an Interesting question...
You take a off the shelf M4gery and mount a 870 shotgun to it with a barrel shorter than the legal 18'', if this shotgun is permanently attached to the reciever of the rifle (welded or simmilar) so that in no way it can be removed without breaking out a hacksaw, would this "combination" be considered a SBW? Because it is permanently attached to a legal non SBR? Or would it be treated like a normal rifle/shotgun? Just curious...

what if you bought say a serbu super shorty and just threw that right on with like a double sided female pic rail adapter

icebones:...You take a off the shelf M4gery and mount a 870 shotgun to it with a barrel shorter than the legal 18'', if this shotgun is permanently attached to the reciever of the rifle (welded or simmilar) so that in no way it can be removed without breaking out a hacksaw, would this "combination" be considered a SBW? Because it is permanently attached to a legal non SBR? Or would it be treated like a normal rifle/shotgun? Just curious...
The 870 that you weld or hot glue to the AR is still a firearm with it's own serial number and NFA rules would still apply. If that 870 has less than an 18"bbl or 26"OAL it needs a $200 SBS tax stamp.

pikid89 what if you bought say a serbu super shorty and just threw that right on with like a double sided female pic rail adapter
If you have a $5 AOW stamp then you just added a stock to an AOW- that's an NFA violation because it is now an SBS needing a $200 stamp. No difference than adding one of those silly Glock adapters to your rifle:
http://cdn5.thefirearmsblog.com/blog/wp-content/uploads/2009/02/glockmount-tm.jpg

ah
thanks for clarifying
for some reason i didnt think of the AR as a stock but it clearly is lol

I figured as much, because the shotgun has its own serial #. Just a wild idea that I had. Thanks