Please don't let this descend into an argument about the laws and regulations regarding legally building/owning AOW/SBS firearms.  I know all about them, and that is not what I'm asking.

Having said that, I have a few extra barrels and mag tubes laying around.  I have always wanted to buy a Serbu Super Shorty, and I've been thinking about just trying to find a bare receiver and building one myself.  If I were to find a receiver I could legally use to do this, how would I go about cutting the mag and barrel down to probably around 8"?  I've cut many barrels down to 18.5", I usually wrap it in masking tape about 1/16" longer than I need, hacksaw it off evenly, and file it to a uniform edge and paint it.  Always works great, but given the length, I've never had to cut the mag down.

To go as short as I want it, I'll have to take the lug off the bottom of the barrel, and cut the mag tube back and somehow cap and thread the end and reattach the lug further back on the barrel.  Has anyone tried anything like this?

I'm asking here rather than the NFA forum because the same procedure could theoretically be used to extend a mag tube to conform to a longer barrel, so I think this is relevant to more than just NFA applications.

On a side note, if anyone has any idea how or where to buy un-assembled receivers for Mossberg 500's, please post it here.

Cut out the middle of the tube, weld end pieces together, file/sand/mill inside so follower does not snag?

(not a gunsmith, nor a machinist)

I'm going off memory here so I could be off

You can shorten the mag tube by either cutting off the end that threads into the receiver, shortening to the right length and then rethreading the receiver end. Or shorten the front end, remove the threaded insert from the front end and reinstall on the tube once shortened to the proper length (I would tend to go this route). I think the factory crimps the insert in place but I would think it could also be welded since the barrel lug will need to be reattached anyways. I think the barrel lug is brazed on at the factory but I would see about welding since I've seen some come off. The cutting of the tube and barrel aren't a big deal, you just need to be, or have the services of, a good welder to reattach the parts properly and in the right places.

Great suggestions!  I was at odds with the lug on the bottom of the barrel, but brazing seems like a good option.  I was afraid to weld it, as I don't want the weld to penetrate into the bore.  Also, brazing can be done at relatively low temperatures, so I shouldn't have to worry about the metal warping at the joint.

To answer your question about virgin receivers, I picked one up ion gunbroker last winter. Seller represented it as a new, unbuilt receiver and based on condition I believe it-but I was replacing a damaged receiver on a bird gun so I didn't have to worry(legally) about whether the claim was true. I THINK to legally build your own AOW you would have to have some type of documentation from the company that it was never in "shotgun" configuration(either virgin receiver or never had a stock installed). I could be wrong. Sounds like an interesting project, keep us posted.
ETA-receiver was $50, and an instate FFL seller so I picked it up FTF. This was cheaper than sending the whole shotgun to Mossberg to have an oversized ejector screw installed, paying for the repair and then paying return shipping(not to mention skipping the 3 week wait time they quoted me)