Can I put a 16" or longer barrel on an SBR lower? I would assume the anwser is yes but we all know what happens when we assume.

yes of course.  You can put any length barrel on an SBR.

yes

Do what now?

My "not quite right" sense Is tingling.
Could you may be show me Where that has either been ruled upon or at least clarified by ATF please?

https://www.atf.gov/firearms/qa/may-short-barrel-sbr-or-sbs-be-replaced-long-barrel-hunting-or-other-purposes-intent

Nothing there about interstate travel, regardless of barrel length.
An SBR lower is an SBR lower regardless of what state you are in. Putting a 16 inch barreled upper on it doesn't make it a non-SBR lower or non-NFA item. The lower is still NFA and is still under all NFA Interstate travel regulations.
That would be like if you had a registered MG lower and took the auto sear out then traveled one state over. Just because it's not capable of full auto doesn't mean it's not a machine gun.

The old ATF SBR/SBS FAQ website used to have the following posted:

Q: May I transfer the receiver of a short-barrel rifle or shotgun to an FFL or to an individual as I would any GCA firearm?  
A: Yes. A weapon that does not meet the definition of a NFA firearm is not subject to the NFA and a possessor or transferor needn't comply with NFA requirements. The firearm is considered a GCA firearm and may be transferred under the provisions of that law.

Q: Who is responsible for notifying the NFA Branch when I transfer the GCA firearm to a FFL or another individual?  
A: There is no requirement that the transferor or transferee of a GCA firearm notify the NFA branch of a transfer or that either party determine whether the firearm was previously registered under the NFA. There is no also no requirement for the registrant or possessor of a NFA firearm to notify ATF of the removal of features that caused the firearm to be subject to the NFA; however, ATF recommends the owner notify the NFA Branch in writing if a firearm is permanently removed from the NFA.

Q: What is the registered part of a Short Barreled Rifle (SBR) or Short Barreled Shotgun (SBS)?  
A: While a receiver alone may be classified as a firearm under the Gun Control Act (GCA), SBRs and SBSs are classified in totality under the National Firearms Act (NFA). A firearm that meets the definition of a SBR consists of a rifle that has a barrel less than 16 inches in length. A SBS consists of a shotgun that has a barrel less than 18 inches in length. The serialized receiver is recorded for registration in the National Firearms Registration and Transfer Record (NFRTR).

Q: I possess a properly registered SBR or SBS. I intend to strip the receiver and remove the barrel prior to selling the receiver. Is the bare receiver still subject to regulation under the NFA as a SBR or SBS?
A: A stripped receiver without a barrel does not meet the definition of a SBR or SBS under the NFA. Although the previously registered firearm would remain registered unless the possessor notified the NFA Branch of the change, there is no provision in statute or regulation requiring registration of a firearm without a barrel because its physical characteristics would make it only a GCA firearm pursuant to 18 U.S.C. § 921(a)(3)(B). If the subsequent owner buys the receiver as a GCA firearm and installs a barrel less than 16 inches in length (SBR) or 18 inches in length (SBS), the firearm would be subject to a $200 making tax and registration under the NFA by the manufacturer or maker of the SBR or SBS. Because registration depends upon the stated intent of the applicant, there is no provision to allow registration of a NFA firearm by anyone other than the maker or manufacturer.  

Q: If I remove the short barrel from the registered SBR or SBS, is the receiver still subject to NFA transfer and possession regulations?  
A: If the possessor retains control over the barrel or other parts required to assemble the SBR or SBS, the firearm would still be subject to NFA transfer and possession regulations. ATF recommends contacting State law enforcement officials to ensure compliance with state and local law.

Q: Does the installation of a barrel over 16 inches in length (SBR) or 18 inches in length (SBS) remove the firearm from the purview of the NFA? If so, is this considered a permanent change?
A: Installation of a barrel greater than 16 inches in length (SBR) or 18 inches in length (SBS) will remove the firearm from the purview of the NFA provided the registrant does not maintain control over the parts necessary to reconfigure the firearm as a SBR or SBS.

Your wrong. A short barrel rifle's status as an NFA is entirely dependent upon its configuration at the moment. If you remove the upper and put a 16"+ it is not an NFA item, therefore it is free to move about. Your machine gun analogy is not correct because the ATF has deemed have a 3rd hole in the lower makes it a machine gun. Even if you remove the sear it is still a machine gun due to the 3rd pin.  

I stand 100% corrected.
Yummy delicious crow.

I would still be worried about a green ATF agent eager to get his first NFA bust on some unsuspecting gun owner who might be doing the right thing by the letter of the law But ultimately winds up in hot water.
I have travel papers for all 3 of my SBR lowers for 10 different states. all the states that border mine, and all the states I would have to go through to get out of state through states that border mine but don't have legal SBR posession I.e. driving around Illinois or Iowa or something like that.

If I get stopped in Illinois with a registered SBR, regardless of the barrel length it has on it, I find it hard to believe it wouldn't be a huge pain in my butt to drive away without experiencing something less than legal that might occur. Idk if suppressors are covered under that but it would be even worse if I had one of those with me.

Wait........you think ATF agents walk around with list of SBR serial numbers?

You own three SBR's as well as silencers and aren't the least bit familiar with ATF NFA regs?

Have you actually read that Form 5320.20? You need to do that before you go to sleep.

If you get stopped in Illinois with an SBR receiver, but with a 16"+ upper on it, how are they going to know that it is an SBR lower? It will appear to be a non-SBR with some additional engraving on it. There is not a magic NFA database that they can run the serial number.

I suppose an ATF agent might have access to NFA registry, but an ATF agent wouldn't be making routine stops.

If you get pulled over for what?  A routine traffic stop is not going to make them search your car for guns.

Assuming they did have a reason to run your serial numbers, they would run it against the stolen firearms database. PD's do not have access to the NFA database.

Deleted.

I wasn't suggesting that in the unlikely event of a routine traffic stop ending in an above average intelligence LEO discovering an SBR lower traveling through a state where A. SBR's aren't legal and 2. AR15's as a whole aren't legal the officer would then call in an ATF agent (Like a good neighbor, ATF is there *POOF*) who would arrest you for unlawfully transporting a registered SBR lower across said state lines.
I WAS suggesting however that it could be a pain in the butt explaining it all without more than a few people being called.

Once again, why would said above average intelligence LEO suspect it is an SBR lower in the first place? Because it has a bit of extra engraving? If the LEO is in fact above average intelligence and knew what the signifigance of said engraving was for, then the above average intelligence LEO would probably know that A) it is not under the NFA purview because it is not configured as an SBR at that time, or B) would know where to look up said regulations.

One would assume. But less likely things have happened.

It is not an SBR if the SB is not on it. It is just a rifle like any other title 1 rifle. The fact that it has it's serial number listed in the NFA database as an SBR is immaterial if it has +16" barrel on it.

Name an instance where THIS situation has happened.

*shrug* I don't know. I'm just saying it could happen. It takes me 5 minutes to fill out travel papers on a Saturday and mail them for free in mid October. They arrive in mid December and I'm covered for the entire next year.  Call me what you will, but my bases are covered.

Well I think you might want to plan for Nibiru showing up if you are planning for the instance stated above, because both are about the same likelihood of occurring.

Don't read too much into things.  

I was at my local range the other day with a guy that had one of those stock things for his Glock.  He was having a blast and letting friends shoot it etc.  I overheard him tell a guy there about it and that he had ordered it over the internet a few years ago and that the guy could do the same thing.

I asked the guy if he had filled out any paperwork or anything for it and he said, "no, you don't have to because you can just order it.".  I asked him if he knew that he had made a short barreled rifle and he looked at me like I had 3 heads and said, "no, its still a pistol, I just put a stock on it.  I take it everywhere with me and shoot it all the time."

So if this guy (who is completely ignorant of said laws and will probably end up in jail somewhere eventually) hadn't got fingered yet, just think about the likelihood that you will even get approached even with all of your ducks in a row.