Constructive Possession: The NFA Trap You Didn’t Know You Fell Into

You don’t need to pull the trigger. You don’t even need to assemble anything. Under federal law, simply possessing certain firearm parts together—even if they’re stored separately—can be enough to land you in serious legal trouble.

This is the concept of constructive possession, and it’s one of the most misunderstood—and aggressively enforced—aspects of the National Firearms Act (NFA). For gun owners, builders, and hobbyists, knowing what not to store together is just as important as knowing how to build safely.

In this article, we’ll break down what constructive possession means, how the ATF interprets it, which configurations can trigger violations, and what you can do to protect yourself from an accidental felony.

What is Constructive Possession?

Constructive possession is a legal doctrine used by the ATF and federal courts to prosecute individuals not for what they’ve assembled, but for what they could assemble based on the parts they own. Under this concept, you can be considered “in possession” of an unregistered NFA item—like a short-barreled rifle (SBR) or machine gun—even if it isn’t built, as long as:

You own all (or most) of the components required to assemble it, and
There’s no lawful use or configuration for those parts outside of an NFA item

In other words, you don’t have to put the parts together. Just having them in your home, truck, or storage unit could be enough to be charged.

For example, if you own:

A pistol lower receiver
A 10.5″ barrel
A rifle stock

…but don’t have an NFA tax stamp for a short-barreled rifle, the ATF could argue that you are in constructive possession of an illegal SBR—even if the items are stored separately or never assembled.

The Legal Pretext

Courts look at intent and capability. If your collection of parts suggests you could readily assemble an unregistered NFA firearm—and have no lawful reason to own those parts in that combination—you could be found guilty of possessing an illegal weapon, even if it’s still in pieces.

Real-World Examples of Constructive Possession

Example 1: The Box of Parts

In the most basic example, let’s consider that you have purchased all of the parts to assemble your first AR: a semi-auto Mk18 clone (a carbine with a 10.3″ barrel). But, you bought all of the components before you realized that you can’t own an SBR without a tax stamp. The parts still sit in the 37 Brownells boxes them came in, stacked in your bedroom closet. Because you have the components to build an SBR, and nothing else that those parts can be used on, you are guilty of a felony. Even if you have since submitted your Form 1 to get a tax stamp for your to-be SBR, you will go to jail if caught.

Example 2: Any Pistol Upper + Any Carbine/Rifle Lower

Let’s say you have a loose pistol upper receiver sitting in one room of your house. Let’s say you have a loose carbine lower receiver in another. In the eyes of the law, you can be found guilty of intent to assemble an SBR. Even if you have never slapped that pistol upper on that carbine lower. Even if the thought to do so has never crossed your mind. You could, and that’s enough. And given that the pistol upper is floating around the house without its pistol lower mate (i.e. it is not part of an assembled pistol), you are, therefore, even more likely to build an SBR. Pre-crime. Go to jail. Do not pass go. Do not collect $200.

Example 3: The Boy Scout

Let’s say that you’ve decided that you don’t want to deal with the pistol brace drama and have disposed of a pistol brace that was previously installed on an AR pistol. Now you have your brace-less pistol outfitted with a carbine receiver extension sitting in your gun safe. Let’s say you are also building another AR; a carbine (classified as a rifle) this time. Right next to that gun safe, you have your brand new Magpul CTR buttstock for your new build. “Constructive Possession” to the rescue. Because you still have that naked carbine receiver extension on your AR pistol searching for a buttstock, and because you have a loose buttstock that is compatible with that carbine receiver extension, you have the partially cooked ingredients to make an NFA Title II weapon without the appropriate tax stamp, and therefore, you can be found guilty of a felony. Way to go, Boy Scout.

Example 4: The Curious Cat

You’ve always wanted to shoot a machine gun. Well, you just found and downloaded the blueprint for a lightning link online. This nifty design allows you to transform a piece of sheet metal into a device that overrides the semi-auto mechanism in an AR. Let’s say you also have the materials to build the lightning link—some sheet metal, a mill or pair of shears, etc. Well, if you own an AR-15, now you can be charged with intent to manufacture an illegal machine gun…even if you don’t manufacture the device.

Example 5: The Keep-It-Quiet Cowboy

Did you know that you can buy a thread adapter to convert an oil filter into a suppressor? Did you know that the end cap of a “solvent trap” (effectively an unfinished suppressor) can be drilled to convert it into a suppressor? Super cool, right?

About as cool as cold steel on your wrists. Possession of these devices constitutes intent or possession of an unregistered suppressor. If you don’t have a tax stamp, clear your schedule for the next 10 years.

How Courts and the ATF Determine "Intent"

Lawyers are crafty. They can make a case for the most absurd charge and convince a judge or jury that you are guilty of it. There are certain conditions that help build a case against you.

Access and proximity to parts: If you have a pistol AR with a bare carbine buffer tube and a carbine buttstock sitting next to it, the prosecutor will have a much easier time building a case against you than if you store the buttstock in an offsite storage unit.
Statements or posts online: If you brag about your new SBR on social media or a discussion board, you are not helping yourself. If you post a video of your lightning link machine gun or “solvent trap” suppressor on YouTube, they have all the evidence they need.
Patterns of purchases: If you buy everything you need to build an SBR over the course of three days, it doesn’t look good for you.
Lack of non-NFA use: If you only own an AR pistol and buy a carbine buffer tube and stock, its easy to see where you’re going.

I am in a Constructive Possession Situation: What Should I Do?

First, don’t tell us! We don’t care and we don’t want to know. We also recommend that you don’t tell anyone else and move to the next step.

Second, stop bending or breaking the law. Remedy the situation before you end up in prison.

If you have a vertical foregrip attached to your pistol, remove it. We understand the benefits of a foregrip on an AR pistol. If you are concerned about blowing your fingers off or the ergonomics of an AR pistol, feel free to swap it out for an angled foregrip, hand stop, finger stop, or barricade stop, none of which currently make your AR pistol any more dangerous.

If you have a device that converts a semi-auto AR into a machine gun or have drilled the “third hole” in your lower receiver, get rid of it. Unless you have a Type 7 FFL and Class 2 SOT, you cannot legally convert a semi-auto AR into a machine gun. If you want a machine gun, you need to buy a transferrable, pre-ban machine gun.

If you have an AR pistol built with a carbine or rifle receiver extension (a.k.a. buffer tube), but do not have a buttstock or pistol brace attached (i.e. a bare buffer tube), you are wading into muddy waters. Technically speaking, if you don’t have a buttstock (or pistol brace) attached to the firearm, the firearm itself is not in violation. However, as described in the scenarios above, if you have a buttstock compatible with that carbine/rifle receiver extension lying around, you could have a problem. And it doesn’t matter if you simply followed the ATF’s guidance in May 2023 by removing your pistol brace; saying “but, you told me to” or “but, I didn’t know” is not a defense for breaking the law, and breaking one law to follow another is still…gasp…breaking the law.

Pistol Upper + Pistol Lower ≠ Prison

If you have a pistol upper, keep it installed on a dedicated pistol lower.

That means a lower equipped with a pistol buffer tube or (for the time-being) a carbine buffer tube and a pistol brace. As ridiculous as it sounds to anyone who knows their way around an AR, having the pistol upper installed onto a pistol lower takes some of the perceived intent out of the question (you have to put forth twice as much effort to break the law, even if it takes 10 seconds instead of 5). This doesn’t mean that you will be absolved of “constructive possession” for this or any other potential violation. But you will have an easier time explaining yourself.

Pistol Upper + Carbine/Rifle Lower = Prison

Now, if you have a pistol upper, a carbine lower (with buttstock), and no pistol lower, you have a bit of a problem. There is no acceptable explanation for having a pistol upper, a carbine lower, and no pistol lower in sight, other than an intent to assemble an SBR. If you have not registered a lower receiver as an SBR, you are guilty of “constructive possession”. Don’t panic if you had no idea, BUT TAKE IMMEDIATE ACTION. You have options.

If you really want an SBR, begin the process to register the lower receiver as an SBR and do one of the following immediately (you cannot legally build or possess an SBR before you receive your tax stamp, so undo it now!).

Option 1: Make it a Pistol

Convert the carbine/rifle lower into a pistol lower, if legally permitted¹. If acceptable to convert the carbine/rifle lower into a pistol lower¹, you may either remove the buttstock (and get it out of the house) or better yet, replace the carbine/rifle receiver extension with a pistol receiver extension. If you have to wait for a pistol receiver extension, at least remove the buttstock and get it out of the house.

1: Note that if your receiver was manufactured as an assembled carbine or rifle, you cannot legally convert it to a pistol. If your receiver was purchased as a stripped or assembled receiver (and not part of a complete firearm) or as an assembled pistol, you can build it into whatever you want. Put simply: pistols can become carbines/rifles; carbines/rifles cannot become pistols. However, even if you are allowed to convert the “accidental SBR” into a pistol, you MUST ALSO FOLLOW YOUR STATE AND LOCAL LAWS. Your state and/or municipality may have requirements for construction, purchase, or possession of pistols. Just because you aren’t heading to a federal prison doesn’t mean you won’t be heading to prison…

Option 2: Make it a Carbine or Rifle

Convert the pistol upper into a carbine/rifle upper. You can swap the pistol-length barrel out for a longer barrel that meets the criteria for a rifle barrel (i.e. at least 16″ long) and be out of troubled waters (please check our Handguard Length article, for additional considerations). If you cannot do this immediately (e.g. you have to wait to receive a longer barrel), at least remove the pistol-length barrel immediately. If you cannot do this immediately (e.g. you don’t have the barrel nut wrench needed to remove the pistol barrel), we recommend that you immediately 1) remove the carbine buttstock and get it (and any other loose buttstocks) out of the house, or 2) separate the receivers and remove the pistol upper from the same house as the carbine lower (and any other non-pistol lowers) until you can remove the pistol-length barrel, to prevent any perception of intent.

Option 3: Build a New Pistol Lower

Build a pistol lower to put your pistol upper onto (e.g. if you can’t legally convert the carbine/rifle lower into a pistol lower1). If you cannot do this immediately (e.g. you have to wait for a permit, a new lower receiver, or parts), we recommend that you immediately either 1) remove the carbine buttstock and get it (and any other loose buttstocks) out of the house, or 2) separate the receivers and remove the pistol upper from the same house as the carbine lower (and any other non-pistol lower) until you have a pistol lower in hand, to prevent any perception of intent.

Moral of the Story

Constructive possession isn’t just a legal technicality—it’s a trap that has landed well-meaning builders in federal court. You don’t need to assemble an SBR or drop in an auto sear to get charged. In the eyes of the ATF, simply having the parts and no lawful configuration can be enough to constitute a felony.

If you’re a hobbyist, builder, or gun owner, staying compliant isn’t just about knowing how to build—it’s about knowing what not to own together, how to store parts properly, and when to submit paperwork. A short barrel, a rifle stock, or an auto sear file might seem harmless on its own—but paired with the wrong receiver, or the wrong intent, it could cost you everything.

Be deliberate. Be informed. And if you’re ever unsure whether a part combination might violate the NFA, assume it does—until you prove otherwise. Because when it comes to constructive possession, ignorance isn’t just dangerous—it’s prosecutable.