I will start this off with the ATFs definitions from the NFA handbook.

A rifle is a firearm designed to be fired from the shoulder and designed to use the energy of an explosive in a fixed cartridge to fire only a single projectile through a rifled barrel for each single pull of the trigger.11 A rifle subject to the NFA has a barrel or barrels of less than 16 inches in length.
The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver
soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.

Weapon made from a rifle. A weapon made from a rifle is a rifle type weapon that has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length.

The overall length of a firearm is the distance between the muzzle of the barrel and the rearmost portion of the weapon measured on a line parallel to the axis of the bore.

To clear up any confusion on "constructive possession" and the belief that there is no such thing. There is established case law.

In United States v. Thompson/Center Arms Company, 504 U.S. 505 (1992), the United States Supreme Court examined whether a short-barreled rifle was “made” under the NFA when a carbine-conversion kit consisting of a single-shot “Contender” pistol was designed so that its handle and barrel could be removed from its receiver, and was packaged with a 21-inch barrel, a rifle stock, and a wooden fore-end. The Court held that, where aggregated parts could convert a pistol into either a regulated short-barreled rifle, or an unregulated rifle with a barrel of 16 inches or more in length, the NFA was ambiguous and applied the “rule of lenity” (i.e., ambiguities in criminal statutes should be resolved in favor of the defendant) so that the pistol and carbine kit, when packaged together, were not considered a “short-barreled rifle” for purposes of the NFA.

However, the Court also explained that an NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel); or (b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a long-barreled rifle and attachable short barrel). Id. at 511-13.

ATF SBR Ruling 2011-4

So I just want to make sure I have this right....

For every NFA length upper (less than 16) you must have it attached to either an engraved SBR lower, or a pistol lower. You cant just have the upper laying around correct?

I build a pistol lower for every <16 barreled upper I get. The way I read it, that makes it good to go.

My 2cents anyway.

The way I play, I have at least one >16" upper for every lower with a stock. Having more <16" uppers than pistol/engraved SBR lowers is okay as long as you don't have a rifle lower with no partner.

You are correct. You can't have a "loose" short upper laying around.

(b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a long-barreled rifle and attachable short barrel)

Per this ruling, having a "loose" short upper, no matter how many complete rifles you have is possession. Put them on pistol lowers, don't tempt fate.

I don't push people to do what makes them uncomfortable.

The key is here. Once you have a pistol lower, or engraved SBR lower and stamp, you have a legal useful purpose.

I keep seeing pistil stamped lower in this topic. My understanding was a new unassembled lower is considered a pistol lower until a stock is added. At which point I can no longer be a pistol lower. Only an sbr.

Is owning a pistol upper and new lower assembled as a pistol, but not owning any sbr, tempting fate or pushing the rules of the nfa?

Based in everything in this post, no. Per prior threads I've read on this very forum yes. So which one is it?

No, a bare receiver is a bare receiver. It is neither a rifle nor pistol until assembled into a configuration that would classify it as such.

Owning a "short barreled" upper receiver and a matching pistol configured receiver (no stock) is legal. Owning that same upper with only rifle configured receivers is illegal e.g. constructive possession.

The same upper with a bare/stripped receiver is legal as long as you do not possess an unattached rifle stock for a lower receiver simultaneously...constructive possession.

If you intend to build an AR pistol, then purchase the receiver complete or build it as a pistol BEFORE buying an upper for it. Way too many people buy the upper first and are surprised to find out that they are now in jeopardy.

This is what I was looking for.

One more. Tac solutions SBX barrel

Is doing the same thing to an AR any different?

The TACSOL barrel shroud is welded on so it is not considered a "short barrel".

If you weld a flash hider to an AR short barrel that increases overall length to 16" then it is no longer a "short barrel".

Cool thanks for the clarity.

You can have a loose SBR upper laying around as long as you don't have a loose non-SBR lower around. If all your non-SBR lowers have non-SBR uppers on them you can have as many loose SBR uppers as you want.

Do you have a shotgun and a hacksaw? Then how come you aren't guilty of 'constructive possession' to build a SBS? Same principle.

You are incorrect. The ruling is pretty clear on this topic:

However, the Court also explained that an NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel); or (b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a long-barreled rifle and attachable short barrel). Id. at 511-13.

Your analogy of "hacksaw/shotgun" is false. The hacksaw has a purpose OTHER than making a weapon e.g. it isn't a firearm and no one would confuse it as part of one.

You know, you may be correct. But then again, you may not be. Do you want to litigate it in Federal Court to get an answer? I'm an attorney and I have an AR. I wouldn't have a SBR upper in my house. I always advise my clients to be very conservative when it comes to NFA items. Why throw the dice?

10 years in prison and $250K fine vs. $200 stamp and 180 day wait. Pretty simple math there.

You're right, one should have a SBR lower to have a SBR upper in the house. But if you have multiple ARs, as long as you have at least one registered lower and no loose unregistered lowers you can have extra SBR uppers. Good point, Basenji.

I don't make my decisions in life depending on how expensive the consequences are. I make them based on the legality of it.

180 days... if only!

Ok so I CAN NOT have a short barrel (pistol) upper and a 16" rifle upper with one lower that I swap back and forth. Changing the lower from a rifle lower with a stock to a pistol lower with no stock. I should have a pistol lower (no stock) and a rifle lower (with stock).

I also read somewhere that you can not have a foregrip on a pistol lower, is that true?

Looks correct on all points to me.

Learn new stuff everyday. Glad I did my research before ordering my parts.

You CAN if the lower is registered as a SBR with the ATF. It is best not to have an AR pistol at all if you have an AR rifle.

Correct, you CANNOT put a foregrip (but and angled foregrip like Magpul is okay).

What? That's not sound advice. It is perfectly legal to own both an AR rifle and an AR pistol.

It is perfectly sound advice. This discussion proves it out. The ATF has continued to give conflicting guidance over the years and have left the community in constant doubt about what is and is not legal.

About the only part of the community that is NOT in some possible conflict with the all of the "guidance" on these is the 07/02 FFL/SOTs (like me). We are good no matter which way a member of the law enforcement/judiciary interprets it since our licensing and status allow us to have all the pieces are parts for manufacturing purposes.

I stand by what I said, best not to have both of them around.