Okay, this question isn't as easy as the title makes it out to be; what I have is truly a rare situation, where I have an SR-25 patterned lower receiver that has been registered & transferred as a 'Shotgun' chambered in 12ga (it has such a corresponding upper).

Now, I want to attach my rifled .308 upper to this lower. Mechanically it works of course, but legally what is the barrel length restriction? I'm going back down to a sub 0.5" bore size rifled barrel, so whether it's now a DD isn't to be called into question, but is my legal barrel length minimum still 18", or can I put a 16" on it?

Is it a rifled barrel that fires fixed ammunition and has a shoulder stock?
If so, I see this to be no different than a rifle or SBR that you want to put a shotgun barrel onto.
Must follow 16" for rifle and 18" for shotgun.

Loading 308 into a 12 gauge magazine well, will be interesting

It sounds like it's a .308 lower that has a conversion upper in 12ga. and either special or converted mags to feed the shotgun ammo.  Here is one example of such a thing:

https://www.thefirearmblog.com/blog/2014/01/19/gun-review-intrepid-tactical-solutions-ras-12-shotgun/

And to agree with the first post, SBS and SBR are different things, and having one stamp does not allow you to configure the firearm into the other.  So you could make it into a rifle that shoots any centerfire cartridge with a rifled barrel, but it would have to be in a title 1 configuration so barrel would have to be 16" or more as a rifle.

Okay, so for clarity's sake, I have this thing just as a Title I firearm, namely a 'Shotgun'. Has not been registered as anything other than such, its 12ga upper has a barrel that's at least 20" long. So I can put a rifled upper on it bearing a barrel just 16" long, even though the lower (the firearm) is a 'Shotgun'?

Did it come from the manufacturer as a shotgun, as a rifle, or as just a lower?

I guess if it started out in it's first complete configuration as a shotgun, then the argument could be made that having a 16" rifle cartridge upper would constitute a firearm "made from a shotgun (whether by alteration, modification or otherwise)" and could legally consider it to be an SBS with the barrel being under 18".

Sort of a "first a shotgun, always a shotgun" like the "first a rifle, always a rifle" concept.  But where it get's weird is that with the rifle upper, it's legally not a shotgun anymore because of the rifled bore and bore diameter.  I'm actually not really certain on this one...

That's what I was concerned about. It did start out its life as a shotgun, I think I made that part abundantly clear already. So putting anything less than 18", whether it's rifled or not would technically be creation of an SBS, I would agree with that opinion.

Yea I see that now, I think I took "registered and transferred" to mean it was an SBS, hence my confusion.

The lower is not a shotgun. Only the complete firearm is a shotgun. Remove the upper, the lower is just another lower. Thus you can put any non-NFA rifle upper on it and now, the completed firearm is a rifle.

Logically I agree.  Unfortunately government is not required to be logical, so the BATFE could in theory disagree.  Lowers are only ~ $30, so I would simply spend a little cash and eliminate the uncertainty.

Government agrees, DOJ agrees, BATFE agrees. They have put in in writing multiple times. There is no uncertainty. It is universally accepted.

Unfortunately the lowest I've seen mil-spec lowers 'out-the-door' these days is $35 before tax, shipping, and transfer fees. And if you read closely here, you'll know the lower in question is actually an AR-10 lower...which are like $100+ at least, stripped.

By the time I'd be finished putting that second one together I may be near $200 or well over. Not cheap, nor am I cheap, but I could get a brand new shotgun for that $$$. I want to make financially unwasteful decisions, lol.