Sorry that I'm late to the party. Hopefully I can try to straighten some of this out because there are some blatent errors.
First, the process for owning a silencer (or short barrel) was talking about was mostly incorrect.
First, the paperwork submitted to the ATF requires the serial number, make, model, caliber, manufacturer, manufacturers location listed, length, etc. (this also applies to silencers)
So, you (or at least your class 3 dealer) must have the silencer on hand so you would have this information to be able to write it down. The class 3 manufacturer maintains possession of the silencer until the paperwork comes back approved, and at that time, the buyer can take it home.
You (nor your class 3 dealer) do not have to have a barrel on hand when you do the paperwork for a short barrel shotgun if you are building it on a form 1, although you would need the reciever you're going to register for the required info (make, model, etc) and you will also have to engrave your name (or trust name), city, state somewhere on the receiver since when the paperwork is approved, legally and technically, you'll be the manufacturer of the weapon since it will be registered to you as your work.
If you buy an existing (or factory built) short barrel shotgun, that would go on a Form 4, and the class 3 dealer would maintain possession of it until the paperwork came back approved.
But this topic sounds like they're going to build it themself on a form 1 from a regular shotgun they own, so this is the process.
You can either order or cut the barrel yourself after the paperwork comes back approved (tax stamp). No special paperwork is required to order the barrel itself because it is not a regulated item by itself, although you can't make a short barrel shotgun without the paperwork, less it's an instant felony and court costs and fine. And if you order the barrel before you get the paperwork, I recommend storing it at a trusted location off site from where you keep your gun for the "constructive intent" clause that has already been mentioned. But it would be absolutely best to wait to order the barrel after you get the paperwork is the safest/best way.
To build (or modify an existing weapon into a short barrel shotgun) on a form 1, you fill out all of this in duplicate:
Form 1 (application to make a firearm)
Certificate of Compliance
FD-258 (Fingerprint cards)
2x2 frontal passport photo
and $200 check
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And as for the barrel length, you can go with 14 inches. Just don't make it shorter than what you write on the paperwork for barrel length. You can make it longer every day of the week, just not shorter. To make it simple, measure (estimate) the barrel length from the front of the reciever to the front of the mag tube. Don't measure any barrel that would be inside the receiver.
Also, it will ask for an overall length of the weapon. measure from the back of the stock to the barrel length you plan for it to be.
I hope this helps. If you have any more questions, just ask.
A lot of people think that owning NFA weapons automatically gets you on watchlists.
With having been through the system about 9 times in total (and currently owning 5 NFA weapons), I have yet had a single problem from it.
You get on more watchlists posting on a gun forum than owning them.
Same for buying ammo and reloading supplies.
Go figure.
Thats funny! Awesome, and funny! 
so I am going to stay with the 18.5 Heavy A1 barrel. Oh well lots of research for nothing. Now who makes a 2 shot extension that is the same length as the 18.5 barrel? :twisted: