Semiautomatic AR with a 16" barrel, classified as a regular rifle.
Semiautomatic AR with a 14.5" barrel classified as a NFA weapon.
Semiautomatic AR with an 11.5" barrel with a welded on 4.5" flash hider classified as a regular rifle.
Semiautomatic AR with an 11.5" barrel and a stabilizer brace classified as a pistol.
Semiautomatic AR with an 11.5 barrel and a collapsible stock classified as a NFA weapon.
It's all arbitrary and makes no sense.
It makes perfect sense in terms of each and every decision made based on a narrow read of the law.
However, it's that process of using "a narrow read of the law" that creates this kind of massive inconsistency in the overall law as the intent is lost.
The original intent goes back to the National Firearms Act of 1934, when the big issue was gangsters carrying full auto weapons, sawed off rifles, and shot guns, etc.
Clyde Barrow for example was fond of carrying a sawed off M1918 BAR. He only carried to and from the car to the bank, and concealment wasn't an issue. Sawing it off made it slightly handier.
However if we fast forward to anytime in the last 40 years, we find that criminals are just not using weapons that fall under the NFA in crimes in anything other than decimal dust numbers. That's because they are generally too large to conceal well and too heavy to carry all day long.
In that regard, the intent of the law in 1934 and the reasons for it, are no longer valid or applicable.
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Let's look at your examples individually:
Semiautomatic AR with a 16" barrel, classified as a regular rifle.
That's based on the criteria established in 1934. That number was picked due to opinion at the time that 16" or longer made it suitably hard to conceal.
Semiautomatic AR with a 14.5" barrel classified as a NFA weapon.
This is based on the 1934 criteria, which was a judgement call at the time. Would 1.5" make a big difference? Probably not. Had the nation been awash in 14.5" Model 94s in 30-30 (i.e had Winchester chosen 14.5" as their standard trapper carbine barrel length rather than 16"), we'd probably have a 14.5" limit rather than 16".
Semiautomatic AR with an 11.5" barrel with a welded on 4.5" flash hider classified as a regular rifle.
Again the idea is total barrel length of at least 16" and blind pinning or silver soldering a muzzle device on a barrel, (so that it cannot be readily removed with common hand tools) was a legal determination that a permanently attached muzzle device that brought the total length to 16" met the intent of the law (and it does).
Semiautomatic AR with an 11.5" barrel and a stabilizer brace classified as a pistol.
As noted above this was based on the ATF deciding that what makes a pistol is designing it to be fired with one hand, and if a brace improves that function it is allowable. Also as noted above the logic used in that narrow read of the law was seriously flawed.
First these "braces" all look a lot like like stocks and can be used like stocks.
Second, the pretense used was that these braces would allow shooters with disabilities to shoot AR-15 pistols. However that ignores the reality that AR-15 pistols are so heavy already that they are not practical for non disabled people to shoot one handed. That was in fact the issue at hand. If you add a brace to an AR-15 pistol, it suddenly becomes massively more practical. The request for the use of a pistol brace had nothing at all to do with disability other than window dressing and using the ADA as a lever to get ATF approval.
But none the less ATF said "yes" using a narrow read of the law that violated the original overall intent of the NFA of 1934.
Semiautomatic AR with an 11.5 barrel and a collapsible stock classified as a NFA weapon.
This so also firmly based in the criteria established in the NFA of 1934, in this case a limit of not less than 26" for overall length of a rifle or shotgun.
That's not even the weird example.
The UZi carbine with a 16" barrel and a folding stock is legal under the NFA given how the ATF requires the measurements to be made. They'll allow a folding stock carbine like the Uzi carbine with the folding stock to be measured with the stock in the fully extended position. It's over 26" in that configuration, but well under 26" when it's folded.
However, if you have an UZI carbine, remove the folding stock and install the older style removable wooded stock, it's suddenly an NFA weapon as the ATF requires the measurement to be made with the stock removed. It's only about 1" shorter than the folding stock version, which is again well under 26" - but ATF rules it as an NFA weapon now, because of the fine print in how they require measurements to be made.
As a result, in order for your wood stocked Uzi carbine to avoid an NFA registration requirement, you need to modify the attachment so that it requires machine screws to be removed to detach the stock. Since you now have to use tools to remove the stock, it's suddenly not an NFA item again.
That's what happens when you use a narrow read of the law and lose sight of the larger intent of the law and/or the lose sight of other differences that have no real distinction.
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The larger issue however isn't how each of these falls in or out of the NFA of 1934, but rather whether the NFA of 1934 really has any utility any more.
In the first place the NFA allowed for registered NFA items with a $200 tax stamp. $200 in 1934 had the same purchasing power in 1934 as $3884 has in $2020. As written, the $200 was meant to be a significant barrier to people registering NFA items. That was the intent. Over time not raising that dollar amount has undermined the intent.
However, more importantly the NFA of 1934 was designed to prevent NFA weapons from being used in crimes.
In 2020, the reality is that since 1934 there have been only two crimes committed with legally registered NFA weapons and one of those was committed by a police officer using a weapon registered to the police department.
That supports the concept that the size of the tax stamp fee has no bearing on whether they are used in crimes. "Affordability" and the related numbers in circulation have no bearing on the use of legally registered NFA items in crimes.
Even illegal items that are not registered under the NFA are rarely used in crimes. Long guns of any kind account for less than 3% of all homicides. Military style assault weapons are less than 1% of the total, and items covered by the NFA are below 0.1%.
In the rare cases where an NFA weapon is used in a crime, the offender is almost never convicted. That's because any weapons charges usually go away under a plea agreement to get a conviction for the primary offense.
That means that nearly all instances where someone is convicted of an NFA offense are status offenses where the offender came to the attention of law enforcement and/or ATF for other reasons. A DUI and an inventory search of the vehicle for example might turn up an unregistered suppressor. That misdemeanor first DUI is suddenly a major felony that won't go away in a plea bargain for a DUI conviction (that is already in the bag anyway). Most prosecutors are not going to let an NFA violation slide when it's far larger than the crime that got the person stopped by law enforcement.
At some point we need to consider whether it makes sense anymore to have items like suppressors included in the NFA of 1934 when they are not used in crimes, but do have great utility - such as suppressors that reduce noise, protect hearing, and are much more neighbor friendly in rural areas.
It clearly doesn't make sense to criminalize people for status offenses for possessing items that do not get used in crimes.
Let me add an example of how you can come to the attention of law enforcement. We had our home security system activate. It was due to a cat knocking a vase off a mantle and setting off the glass break alarm. My wife had also failed to lock the back door, with the result that the police entered and searched the house. They found a faux silencer sitting on a desk that I had removed from a Colt M4 .22 LR (which still has a 16" barrel length without the stupid looking thing). I arrived before the officers left and they questioned me about my "suppressor". I pointed out it was a faux suppressor drilled straight through with no baffles and effectively extended about 1" past the muzzle as it was a slip over design. Had they been less reasonable or understanding of the difference, or perhaps had I not looked so upstanding or not been vetted with a concealed carry permit, I could have been explaining it downtown while in custody.