What Is the Legal Length of An AR Rifle Barrel?

The AR barrel, is a beautiful piece of engineering that forms the cherry on top of the complex cake that is your customized gun. Without your barrel, all those other little modifications become fairly pointless, and your gun practically useless. It’s easy to think of the barrel as a kind of exhaust that facilitates the ejection – at high speed – of your round, but it is actually far more complex than that.

The average life from an AR barrel is somewhere around 20,000 rounds, and each one of those is going to impart the pressure and temperature of the ignition cycle of the round, and then enclose the bullet as it hurtles down the length of the barrel on its way to whatever you have in your sights. That explosion-containment is pretty impressive anyway, but made even more so if you happen to let loose on full auto, which will see your AR potentially hit a fire rate of over 400 rounds per minute! And that means a lot of heat if you happen to have a KCI AR-15 .223/5.56mm 100-Round Gen 2 Drum Magazine fitted.

In fact, if you have multiple magazines of that type, you can pass a hell of a lot of rounds through that barrel in a fairly short time, and tests have shown that the breach and lower end of the barrel can approach melting temperatures at close to 600 rounds per minute! That’s something to think about. And if you are more inclined to the AR-10 and a round like the .308, you have an overall larger charge in each bullet and even simple physics tells you that the amount of heat generated is greater than that of the smaller .223 round, so you will be pouring even more heat into the system! It’s probably best not to try such rapid firing with any round larger than the .308 as heat will build up very quickly and the shock effects on the structure of the gun could be catastrophic.

But your barrel is also king when it comes to accuracy, particularly over distance; the wonder that is rifling keeps your round on the right path for longer, and the greater the barrel (and rifling) length, the longer that happens for. Now imagine not having any discernible barrel and see just how well your bullet flies. This is a key aspect to the properties of barrel and one that is impacted by the length.

The longer your barrel, the greater the initial velocity of the bullet, and the greater the amount of spin imparted on it by the rifling so, subsequently, the greater its stability as it leaves the end of the barrel and the longer it will stay on a level path. Some might point out the potential for a longer barrel to ‘whip’ with the firing of a round, but this is in reality very small and really only an issue for match shooters who require incredible accuracy and repeatability. Therefore, having a longer barrel makes sense if you are concerned about long-distance accuracy, but that still doesn’t answer the question of where they stand in law with the barrel length on an AR.

Many people are less concerned with accuracy and prefer the notion of concealment and the ability to fire rapidly, hence the growing interest in so called AR pistols. Okay, so by definition, a handgun is any firearm which has a short stock and is designed to be held and fired by the use of a single hand. But an AR pistol is simply one of those with a shoulder stock attached, which allows concealment and maneuverability in confined spaces. AR pistols – which aren’t restricted too much by caliber – you can make a .308 version just as well as you can make a .223 version – are hugely popular and much loved by the customization crowd, but they still can’t be as short as some like a Glock 17 and inhabit a notional area between long pistol and short rifle.

Okay….So What’s The Legal Length For An AR Barrel?

An AR pistol is restricted in length by the minimum allowed barrel length for a rifle, and that magic number is 16”. Once you add in the working of the rifle, and a short stock, you are looking at something with an overall length of around 28”. And that’s a very short rifle.

In truth, you can get an AR barrel of 7.5”, but that can’t be legally used if you have a buttstock in place. Congratulations, you have now started to enter the red tape maze that is US weaponry law.