You are asking for trouble.

If you want to own multiple sub-16" uppers for one SBR lower and also own additional AR15 rifles, make sure you have a 16"+ upper for each of the non-SBR rifles. And store the non-SBRs with the 16"+ uppers installed.

Quoted: I don't own any short uppers right now and would have to sell all of my rifle uppers once my form 4 is approved for this to happen. So don't worry, I don't plan on doing anything stupid. I was just wondering. I mabe should have asked the question differently. If I have one complete SBR with one extra SBR upper. Can I have all my complete rifles and one spare rifle lower, that I do not have an upper for right now, all in the same safe and not have to wach my cornhole? So I would have all complete rifles (SBR and regular) but with one extra sub 16" upper and a spare 16"+ rifle lower. I think that this is a possibility since I will only have one SBR that I plan to get several uppers for,  and right now I have a complete lower with no upper. I will not do any thing illegal, but would this look bad? If it does I will just buy another 16"+ upper for the lower before I get any extra uppers for my SBR.

Let me put it this way: People have been arrested, and convicted of felonies, for possessing short uppers and spare lowers without having a 16"+ upper for each spare lower.

The charge is called constructive possession of an unregistered NFA item -- you have all the parts neccessary to build an NFA item, but do not possess parts to assemble them in any legal way.

U.S. v. Thompson/Center Arms Co., - U.S.- (1992)
This is another recent examination of the meaning of the language of the NFA by the Supreme Court, along with Staples. Neither involves constitutional law. In this case the court was called upon to decide what constituted a short barreled rifle. T/C wanted to market a kit consisting of one receiver for their Contender gun, a 16"+ barrel, a >16" barrel, a pistol grip and a shoulder stock. This kit could be used, as intended, to assemble a rifle or a pistol, or it could also be used to assemble a SBR. As it could be so used, ATF decided it was a SBR. T/C made one unit on a Form 1, then sued for a tax refund, claiming it wasn't subject to the NFA. This is the way to challenge such a classification. Doing the thing York or SWD did, in those cases, is an invitation to a prosecution. The Staples case will limit such things, but one can easily lose....Here all that was at stake was money. The court decided that the language of the definition of a SBR was vague, and gave it the reading most favorable to the taxpayer, T/C. They decided the kit was not a SBR, nor was any set of parts where they could be used for a legitimate purpose, even if they could also be used to assemble a SBR. However a SBR fully assembled was also clearly a SBR. Thus the other grey area was a SBR in parts form, like an Uzi carbine and a Uzi SMG barrel. A lower court had held in a prior case that that set of parts was a SBR. The court agreed; that if the parts had only one use, to make a SBR, and a person possessed them all that was a SBR also.

Quoted: What if you have a M16A2 on a form 4 with a 20" barrel and want to put a LMT MRP 10.5" barrel on it but you don't want to permanently change the configuration and notify ATF. Then the ATF decide to check on your gun and they see that you put a collapsible stock on and a 10.5" barrel. Then they say you have to put it back in it's original configuration. Oops now you have a 10.5" upper with non NFA receiver. Is this possible or am I beating a dead horse?

That horse is dead.

BATFE can't "say you have to put it back in its original configuration" as long as you still have the parts to do so -- there is no law against temporary changes in config, and in the U.S., if there is no specific law against something, it is presumed to be legal.

Now, if you have an M16A2 with a 20" barrel and buy a LMT MRP 10.5" barrel, and also happen to own a Title 1 AR15 lower with no accompanying upper, then yes, you have a problem: Under the Thompson precedent, you have all the parts to assemble an unregistered SBR (the AR15 lower + the LMT 10.5" upper) and do not have enough parts to also assemble them in a legal configuration. That equals constructive possession of an unregistered SBR.

Quoted: Quoted: Quoted: What if you have a M16A2 on a form 4 with a 20" barrel and want to put a LMT MRP 10.5" barrel on it but you don't want to permanently change the configuration and notify ATF. Then the ATF decide to check on your gun and they see that you put a collapsible stock on and a 10.5" barrel. Then they say you have to put it back in it's original configuration. Oops now you have a 10.5" upper with non NFA receiver. Is this possible or am I beating a dead horse? That horse is dead. BATFE can't "say you have to put it back in its original configuration" as long as you still have the parts to do so -- there is no law against temporary changes in config, and in the U.S., if there is no specific law against something, it is presumed to be legal. Now, if you have an M16A2 with a 20" barrel and buy a LMT MRP 10.5" barrel, and also happen to own a Title 1 AR15 lower with no accompanying upper, then yes, you have a problem: Under the Thompson precedent, you have all the parts to assemble an unregistered SBR (the AR15 lower + the LMT 10.5" upper) and do not have enough parts to also assemble them in a legal configuration. That equals constructive possession of an unregistered SBR. OK I am going to disagree with Tony_K here. If you have an M16A2 with a 20" upper and purchase a LMT 10.5" upper in conjunction with a spare lower .. you haven't broken any laws AS LONG as the 20" upper resides on the "spare" lower and the 10.5" on the M16

I guess we'll have to agree to disagree, then.

The whole "constructive possession" issue is a grey area -- whether you are considered to be breaking the law depends on the circumstances in which it comes to the attention of LE; the attitude of the investigating officers; the goals of the DA or prosecutor who gets the case; and the political climate where you live. Under the same law, legal in one place can be illegal in another.

In the (red) "as long as" situation you outline above, you have in effect married the 20" upper to the spare lower. If you need or attempt to reconfigure the M16A2 to the specs listed on the Form 4, you are left with a parts combo that is a felony. If you cannot do so, your M16A2 no longer can be restored to its registered configuration, which also is improper.

Given all the variables re: arrest and prosecution, I prefer playing it safe. God, with his Infinite Sense of Humor, must like to watch me sweat, because of the many times I've found myself in unexpected, unlikely situations. So I just avoid situations like this (and keep an attorney on retainer anyway).

As always, Your Mileage May Vary.

Quoted: If you can assemble everything you own in a legal manner, the ATF would have an insurmountable obstacle to proving constructive intent.  You can have a hundred short uppers if you have a single pistol or SBR lower, because you can easily show that everything in your collection can be used legally. Think about it.  If you own a complete SBR and a complete non-SBR, you could theoretically put the shorty upper on the unregistered lower and have an illegal gun.  But ATF would never come after you for owning those rifles, because there is an obvious and legal way to use everything you own.

My experience with ATF, and knowledge of their past practices, differs from yours. In fact, I would have to say that when something is "obvious," that's a guarantee that ATF will take the opposite stance.

Again, YMMV.

Quoted: I'm not talking about someone with NO registered lower.  I have very clearly said that as long as you own ONE pistol/SBR/MG lower you can own as many short uppers as you want.  Kent did NOT have a single pistol/SBR/MG lower, so you have made a straw-man argument.  You have not proved me wrong, only proved that possession of a short upper with NO legal lower is a felony, which we all already knew.

Let's say you are correct.

Do you want to be the test case?

I do not. In fact, I will go to extraordinary lengths to avoid even the most remote possibility of it happening.

Your mileage May Vary.