It’s probably no surprise that your AR-15’s barrel can influence quite a bit in terms of how your rifle performs. From maximum effective range and kinetic energy to optimal ammunition loads and even the legal status of your firearm, there are a lot of factors to consider, and choosing the right barrel can seem like a daunting task. That’s why we’re breaking down some of the most common barrel length options for the AR-15, and detailing the major pros and cons of each one.
Barrel Length and Legality
Before we start talking about barrel lengths in more detail, it’s important to get this part out of the way: the National Firearms Act of 1934 established a legal minimum length of 16 inches for all rifle barrels, measured from breech to muzzle. That length includes permanently attached muzzle devices such as flash hiders or muzzle brakes, and by “permanent” the ATF means either blind pinned and welded, silver soldered, or steel-seam welded. Any barrel that falls under this minimum threshold will require either a Class III tax stamp to be registered as a short-barreled rifle (SBR) or will have to be built in an AR pistol configuration.
Factors to Consider
As is often the case with building an AR-15, there is no “one size fits all” choice when it comes to barrel lengths. The most important factor in making your decision is going to come down to how you intend to use your rifle – an AR that is going to be used as a truck gun or home defense tool will benefit from a shorter, lightweight barrel without sacrificing performance, while people who are interested in precision benchrest shooting or long-range competitive shooting will require the higher velocities that long barrels offer.
Of course, there are an almost endless combination of other variables that can influence your choice of barrel length, from chambering (a cartridge like .300 BLK can achieve 95% of its ballistic potential out of a 9-inch barrel, while it’s not unusual to see a heavier round like 6.5 Grendel paired with a 20-inch or even 24-inch match grade barrel), twist rate, ammo type, barrel profile, and rifling style. Covering all of the interactions between these individual factors would be far beyond the scope of a single article, so for the sake of simplicity we’re going to be looking at the pros and cons of each barrel length in relation to a typical AR-15 chambered in 5.56 / .223.
[To get a brief overview of all the calibers an AR-15 can shoot, check out our AR-15 Calibers guide!]
Common Barrel Lengths
20 Inches or Longer
ArmaLite’s original 1956 design for the AR-15 incorporated a 20-inch barrel and a rifle-length gas tube, and it could even be argued that a 20-inch barrel offers optimized ballistic performance for the 5.56 NATO cartridge – M855 “green tip” projectiles reach the 3,000 fps velocity breakpoint out of a 20-inch barrel, and performance is fairly consistent across a wide range of other loads as well. If you’re looking to make accurate shots in excess of 400 meters, a barrel length of 20 inches or longer is a good way to ensure that velocity is maintained and that the bullet remains stabilized and supersonic for as long as possible.
The other benefit that a barrel of this length offers is an increased sight radius. While this doesn’t mean much to shooters that are using a telescopic sight, the slightly more front-heavy balance of a long-barreled AR-15 also makes it easier to control the “wobble zone” – the appearance of your sights drifting around the target area while aiming, whether you’re using irons, a scope, or a red dot. It’s also worth mentioning that a rifle length gas system makes for a noticeably smoother shooting experience.
Of course, these long barrels have some notable downsides, too. As you’d probably expect, a 20-inch barrel is significantly heavier and less maneuverable than a shorter one – that’s not a dealbreaker if you only plan on taking your rifle from the gun safe to the range and back, but if you’re looking for a home defense gun, a duty rifle, or plan on hunting in thick brush, it can be a real problem.
It’s also worth noting that the higher velocity of a long barrel can also create some potential compatibility issues depending on the twist rate and type of ammo used. While you’re not likely to encounter this issue, a combination of high velocity and fast twist rate can cause some lightweight thin-jacketed bullets to over-stabilize and even possibly disintegrate in flight.
The civilian-standard 16-inch barrel is a solid compromise between the ballistic performance and softer recoil impulse of a 20 inch barrel and the maneuverability of a shorter barrel. The biggest draw of the 16 inch barrel is its versatility – you can freely swap muzzle devices without running afoul of the ATF, and paired with a mid-length gas system or adjustable gas block it is capable of handling an enormous variety of both factory and handloads without running into any reliability issues.
Because it is the most common off-the-shelf AR-15 configuration, there is also slightly more aftermarket support for rifles with 16 inch barrels. You’ll have absolutely no trouble finding the perfect set of handguards for your needs, and you’ll have plenty of real estate for foregrips, lights, and other accessories.
At the end of the day, a 16 inch barrel is a balanced, jack-of-all-trades option that is often recommended for first-time AR owners. This barrel length may not have a specific niche that it excels at, and it does sacrifice a small amount of velocity and effective range compared to longer barrels, but it is highly unlikely that you’ll ever find yourself in a situation where it isn’t good enough for the task at hand.
As we mentioned earlier, a 14.5-inch barrel would normally be a recipe for an unpleasant visit from a federal agent unless you paid for a tax stamp or planned on using it to build an AR pistol, with one major exception: as long as you’ve got a permanently attached muzzle device that brings the total barrel length to at least 16 inches, you’re good to go.
14.5-inch barrels are the standard carbine length used by the U.S. military’s M4 and M4A1 rifles, thanks to the need for a lightweight rifle with a shorter overall length for use in dense urban combat zones where clearing rooms or rapidly deploying out of a vehicle would make longer barrels impractical.
While a 14.5-inch barrel with a pinned and welded flash hider or brake can shave off an extra couple inches overall length compared to a 16-inch barrel (though a number of super-compact brakes and compensators for 16-inch barrels have come onto the market that make this difference much smaller), it does come at the cost of making it much more difficult (and expensive) to make modifications if you decide you want to upgrade or switch out your muzzle device.
If you have an exact build planned out and don’t anticipate changing your mind, going with a 14.5-inch barrel lets you squeeze out a little bit of extra maneuverability and an ounce or two of weight at the cost of a negligible amount of velocity. But if you’re a first-time AR owner and want the option to freely switch out muzzle devices without worrying about legal repercussions or gunsmithing costs, we’d recommend sticking to 16 inches.
10.5 Inches or Shorter
Now we are firmly into SBR and AR pistol territory, with options ranging from 10.5 inches, 8.5 inches, and even 7.5 inches all being readily available from a number of manufacturers. These barrels are designed for one thing, and they do it very well: being extremely compact and easy to maneuver in tight quarters. While you shouldn’t be expecting to make shots at 300+ meters with these barrels, they are more than capable of handling threats within typical engagement distances, which is why they have become so popular in the world of special operations forces and law enforcement units who routinely work in urban environments.
The ergonomic benefits of an SBR definitely come with some notable trade-offs, however, and we’re not just talking the $200 tax stamp. Below 10.5 inches, getting adequate dwell time can be a real issue even with a carbine or pistol-length gas tube, and the rifle may short-stroke as a result. Because a shorter barrel also leads to higher pressures, most shooters opt for a heavier buffer weight to limit the risk of cycling issues and reducing the recoil impulse. Velocity also suffers significantly, and the noise of a short barreled rifle can be painful even with ear protection – in a situation where you’re shooting indoors, anything short of a suppressor puts you at real risk of permanent hearing damage.
The bottom line is that these barrels are excellent for their intended purpose, and they can be a ton of fun to shoot, but they do come with a lot of extra paperwork (filing a Form 20 when you want to take your rifle across state lines is more interaction with the federal government than many shooters are comfortable with) and require more consideration than a more traditional barrel length.
A Note on Accuracy
You’ve probably heard the common wisdom that a longer barrel is inherently more accurate than a shorter one, and while there’s a kernel of truth to that, there are also quite a few misconceptions about the subject.
When people say that a longer barrel is more accurate, what they’re really saying is that they have a higher velocity because there’s more room for the powder to burn. Not only does that higher velocity mean the bullet will travel in a flatter arc, it also means there’s less time for environmental factors such as wind and gravity to influence the trajectory. This becomes even more pronounced at longer distances, especially with a light cartridge like 5.56 that starts to shed velocity pretty rapidly past 300 meters, until eventually it goes subsonic and destabilizes.
That doesn’t mean that the mechanical accuracy of a longer barrel is superior – in fact, longer barrels are more susceptible to what is known as “whip,” which is when the force required to accelerate a bullet into a fast enough spin to stabilize it causes the barrel of the rifle to flex. Comparatively, a shorter, more rigid barrel is less effected by these forces.
But it’s important to remember that barrel length is only a small part of the equation. Ammunition, twist rate, fitting, barrel profile, material, and production process, and of course the skill of the shooter are all going to have more of an effect on your rifle’s accuracy than the length of the barrel. While a longer barrel will require less holdover and provide superior ballistic performance when you’re taking shots at long distances, that won’t mean much to someone who does most of their shooting inside of 100 meters or who wants an AR-15 for personal protection.
Which Barrel Length is the Best?
At the end of the day, we recommend choosing a barrel based on what you intend to use the rifle for – every length has its pros and cons, and none of them excel in every role. 16 inch barrels are the most popular due to their widespread legality and compromise between performance and weight. But the modularity of the AR platform is one of its greatest assets, and one the best parts of owning one is that you can easily and affordably switch between different barrel lengths to suit your needs.
[To learn more about choosing the right AR-15 barrel, check out our article about the Best AR Barrels. Here at Bear Creek Arsenal we offer a huge selection of AR-15 barrels, AR-10 barrels, and AR-9 barrels in 12 different calibers. Make sure to leave a comment below with any questions or insights!]