If you bolt a handgun or a smoothbore AOW to a firearm with a shoulder stock, the handgun/AOW becomes an unregistered SBR/SBS. So yes, if you want to do that, you need to first register the handgun/AOW as an SBR/SBS.

Yes, they are separate firearms, even when connected. Just like a suppressor is a separate item requiring its own registration, even if you weld it to an already registered SBR.

The legal definition of shotguns and rifles list a minimum barrel length and minimum overall length; if these minimums are not met, the shotgun and rifle fall into the SBS/SBR definitions. The legal definition of a machine gun has no minimum barrel length or overall length, and it also is neither a rifle, nor a shotgun, nor a handgun -- it is in its own category. Therefore, the barrel-length and overall-length requirements that apply to a rifle or shotgun do not apply to machine guns.

Bottom line: What you're asking is not do-able.

Quoted: Sorry for the hijack but if you cut down a drilling (I shudder to even type that) or a combo gun (rifle/shotgun) would it be a SBR, SBS or ??

A combination long gun with both rifled and smoothbore barrels does not fit the definition of either a rifle or shotgun; if you cut down the barrels, it would require registration as an AOW, since it is neither an SBR nor an SBS.

I still learn new stuff about NFA every day. I've just been doing this for long enough that I've made most of the common mistakes, learned from 'em, and now I try to help others avoid those landmines.

Anyway ....

Yes, a AOW made from a combination rifle/shotgun is defined in the law as having barrels that are between 12" and 18".

That said, BATFE has in the past approved Form 1s and accepted Form 2s for combo-gun-based AOWs with barrels shorter than 12". I don't know if that is policy, or if it happened by mistake, but they are out there.

A combo gun with barrels shorter than 12" would not neccessarily qualify as a DD -- for instance, what if it were based on a Savage 24 that was .410 and .22RF? It's not an SBR, not an SBS, not a DD (.410 bore is not big enough), not a Title 1 handgun (one of its barrels is smoothbore) ... there is really nothing other than an AOW which it can be.

You could write to BATFE Tech Branch ... but nine times out of 10, Tech Branch bases its decisions on an exclusionary approach: If something does not fit an existing legal Title 1 or Title 2 firearm category, it must be illegal (an approach which, of course, is contrary to our entire legal system -- in the U.S., everything is legal unless it is specifically prohibited by law).

Again, I am not a lawyer and this is not legal advice, but if I wanted to build a combo firearm with barrels less than 12", I would just submit the Form 1 for an AOW listing the barrel length I want, and see what happens. By so doing, I would be making a reasonable attempt to comply with NFA law.

If they decide it belongs in an NFA category other than AOW, I'll just change the info in the "type" box of the Form 1 -- the $200 making tax is the same no matter what type of NFA device they say it is. And if they flat-out reject it as unbuildable, they need to cite the law which prohibits it.