There is no question that Gene Stoner’s masterpiece design, the AR series rifle, is the most configurable weapon on the planet. Millions of copycat versions of this rifle exist in innumerable forms.
From 10” barreled pistol versions to 22” barreled match versions and everything in between, the AR series is the Lego of firearms. It’s a weapon that many build at home with simple hand tools; most anyone with rudimentary skills can successfully assemble an AR rifle .
There are some massive legal pitfalls in putting one of these rifles together, however, if you don’t have a basic understanding of federal law, specifically, the laws and regulations relating to National Firearms Act  (NFA) weapons. Building AR series rifles is a fun and rewarding hobby, but the last thing you want is a stint in federal prison because you unknowingly build an illegal rifle. Remember – ignorance of the law is never an adequate defense. So let’s get started with some basic terminology:
Pistol: A handheld firearm that is designed to be fired by holding a small stock that protrudes under the bore, usually with one hand. Barrel length can be just about anything, which challenges the popularly conceived notion of what a pistol is.
Rifle: A shoulder fired, rifled barrel firearm with an overall length of 26” or greater, even when a folding or collapsible stock is retracted. Minimum barrel length is 16” from breech face to crown.
Okay, so the above seems pretty clear – or so you would think. But like a late night commercial – wait – there’s more! You can absolutely run afoul of federal law with the slightest of modifications to the above definitions.
Here’s how to ensure you violate NFA law on an AR series rifle:
1. Butt Stock on a Pistol: So your shiny new AR pistol with a 10” barrel is a little bit of a handful. For whatever reason, the manufacturer shipped it with a foam cover over the buffer tube. But wait — that’s a normal buffer tube there…one that would easily accept a conventional AR butt stock. Guess what? As soon as you place a butt stock on that pistol, you just created an illegal short -barreled rifle. Why? Simply put, the pistol was designed to be fired with your hand, not from the shoulder. You changed its characteristics to turn it into a shoulder fired weapon with a short barrel, which, by the way is a controlled item that you need to register with ATF and pay a $200 tax stamp on.
2. Fore Grip on a Pistol: Although not on the law books per se, the ATF has ruled that placing a fore grip on a pistol creates a new class of weapon – an AOW (Any Other Weapon), which must also be registered and a fee paid. While it might seem tempting to mount a vertical fore grip to your AR pistol, and while technically the law is silent on this addition, the ATF still takes a dim view of it and in the end, even if you are right, you will have to endure an expensive trial to prove it.
3. Short Barrel on a Rifle: Let’s say that you own both an AR pistol and an AR rifle. Modularity and ease of configuration being what they are, you decide to pop the pins on both, and put the short barrel on the rifle. Of course, as stated before, you just illegally manufactured a short barreled rifle even if the barrel you used was perfectly legal on the pistol just moments ago. This concept is pretty obvious to most people, but there are subtler ways of entrapping yourself with barrel length.
Let’s say you are building an AR and you order a 14.5” rifle barrel and affix it to the AR rifle. You want a 14.5” barrel because that’s what the Army uses, and you want an M4 clone. This is still too short to stay legal, so you add a 1.5” flash hider to stay in the clear. Now you’re at 16” length. OK – right? Possibly not! The ATF requires that all flash hiders, to be counted as part of the overall barrel length, must be pinned and welded or brazed on in a permanent fashion. No using Loc-Tite or just a wrench — the flash hider must be permanent!
Building ARs is a fun hobby, and shooting them is even more fun, but just be aware of the legal ramifications when you build. Just because it snaps together perfectly, doesn’t mean it’s legal!