AR15 barrel length is one of the most hotly contested areas of rifle builds. There are those that would tell you barrel length is a matter of life and death, or that selecting the wrong barrel will result in wide misses even at competitive intermediate ranges.
Mostly, that’s dramatic, though it is important to understand how the length of an AR15 barrel will influence ballistic performance, accuracy, handling, and more. That’s just what this guide will do.
AR15 Barrel Length: A General Overview
For the purpose of this article, we’re going to keep the cartridge constant. There are lots of different popular AR chamberings out there, but so we can compare apples to apples, we’re going to assume that you’re building around the old reliable 5.56 NATO.
That being the case, the two most important things you have to understand about AR15 barrel length are these.
The longer the barrel, the greater the muzzle velocity will be when the bullet exits the muzzle, which will result in a flatter trajectory and greater overall range. A shorter barrel will generate greater bore pressures, and may exhibit less barrel whip, but it will not shoot as fast or as flat as a longer barrel, holding all else equal.
This is important because it does have implications for accuracy, bullet drop and effective range, but it does not paint the entire picture because it leaves weight and handling out of the equation, so we need to peel the layers back and take a closer look, by specific length.
Specific Notes By Barrel Length
For those of you looking for more information on how AR15 barrels of a specific length will perform, here’s a high-level breakdown.
- 20 inch barrels: A 20 inch AR15 barrel has been a standard for a long time. All else being equal, a 20 inch barrel will produce the highest muzzle velocities and the flattest trajectories, offering a ballistic advantage, which is one of the reasons that this barrel length is often chosen by AR shooters who specialize in long-range shooting. There are some shooters that would even argue that the longer barrel shifts the balance point of the rifle forward, which stabilizes it during unsupported shooting. Some shooters prefer this, even though it makes the rifle heavier. That said, the longer, heavier 20 inch barrel will not be as maneuverable, especially in close-quarters, as shorter barrels.
- 18 inch barrels: The 18 inch barrel enjoys popularity in some circles, but it is a bit of a niche option. It performs relatively similarly to AR uppers with 20 inch barrels, in terms of velocity and trajectory, but it also takes a few ounces off the end of the rifle, which some shooters prefer. The other consideration is that the shorter gas system can be a little less reliable in colder weather.
- 16 inch barrels: These are carbine-length barrels that are among the most popular among all AR uppers. They are probably the most common among civilian shooters. They don’t outperform 20 inch barrels; they do produce less muzzle velocity and don’t shoot as flat, but they only lose about 50 yards of effective range. Also, they’re significantly lighter and handle more effectively in close quarters, which contributes to their popularity.
- 5 inch barrels: Once you get below 16 inches, you’re looking at short barrel rifles, which are regulated by the NFA. In the civilian sector, 14.5 inch barrels would likely be more popular if it were not for that. The paperwork you need to do to get an SBR, for most, makes more headaches than its worth. It’s true that a 14.5 inch barrel will be a little lighter and handle a little more nimbly than a 16 inch carbine, but you’ll lose a little bit of muzzle velocity and produce a much louder noise signature, which is a pain, given all the red tape.
- 5 inch barrels: AR uppers with 10.5 inch barrels tread the line between short barrel rifles and AR pistols, or ARPs. The benefit of an AR upper with a 10.5 inch upper is that it’s a lot lighter and will handle better in close quarters than a carbine. The price you pay for that is that it will produce much higher bore pressures and muzzle pop, much louder volume, and it will produce less muzzle velocity and a shorter effective range, paired with more dramatic bullet drop. For close-quarters engagements, this is not necessarily an issue, but it makes ARs with shorter barrels less effective for long-range shooting.
- 7 inch barrels: An AR upper with a 7 inch barrel is an AR15 pistol upper; because it isn’t considered an SBR if it lacks a stock, there’s less regulation surrounding it, which makes it easier to build a micro ARP. However, the tradeoff with respect to muzzle velocity and effective range is even more severe. It will produce a lot of muzzle pop and a loud noise signature and will produce high bore pressures, wearing out the bolt carrier group more quickly. As a result, effective range will be considerably curtailed. The benefit is that ARPs offer much more compact profiles that are easier to conceal and, according to some, which handle better for close-quarters engagements.
- 5 inch barrels: An AR upper with a 5 inch barrel will yield among the smallest of all micro ARPs, especially if you build with a bufferless upper, or use a short buffer tube. The notes for 5 inch barrels and 7 inch barrels are similar. A 5 inch barrel will yield very high bore pressures, will produce a lot of fouling, and will produce inferior muzzle velocities and shorter effective ranges, paired with a lot more bullet drop. That being said, their compact nature and lightweight design makes them popular among some ARP fans for close-quarters defensive engagements.
Again the high-level takeaway is this.
With a longer barrel, you get:
- Lower bore pressures.
- A flatter trajectory resulting from higher muzzle velocity.
- Greater effective range.
- A heavier, more stable rifle.
With a shorter barrel, you get:
- Greater maneuverability.
- Lighter weight.
- Easier concealment.
- Higher bore pressures and a louder noise signature.
- A short effective range.
- Lower muzzle velocity, incurring faster bullet drop.
At the end of the day, the case is neither that a shorter barrel is better than a longer barrel or vice versa. Rather, the best choice depends on your intended use and what you need from the rifle.
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