Of the many rifles available today, the AR-15 stands out as one of the easiest to build and customize. The AR’s upper receiver greatly determines how the rifle performs in different applications, making them one of the more important upgrade options for those wanting to enhance or build a rifle.
Complete upper receivers significantly simplify the process of building a rifle. Coming pre-built and ready to use, they take out the guesswork of assembling small parts and often come with a factory guarantee. Plus, with the dozens of brands producing them, there’s enough variety to ensure that there’s something for everyone.
When shopping for an upper, its length is a crucial factor. Different barrel lengths offer different degrees of performance across a wide range of applications. And more importantly, it can also affect your rifle’s legality. Below, we’re looking at the importance of choosing the right upper receiver for your build and the options to choose from.
Legal Barrel Lengths for AR-15s
Although AR-15 upper receivers themselves aren’t classified as firearms like lower receivers, their barrel length plays a critical role in determining the legality of your finished rifle. According to the ATF, rifles need to have a minimum barrel length of 16-inches to be classified as such. Anything shorter than 16-inches encroaches on NFA territory, turning your rifle into a short-barreled rifle (SBR).
SBRs are an NFA-regulated item, meaning you must undergo a separate screening check from the ATF and purchase an added $200 tax stamp for your rifle to own one. The approval process can be quite lengthy as well, with it taking anywhere from a matter of days or weeks, to upwards of several months to complete. However, this doesn’t mean that you can’t use uppers shorter than 16-inches. You can alternatively opt to build an AR pistol with a shorter upper, or you can pin and weld a muzzle device to your upper to extend its barrel length.
We’ll go more in-depth on this later, but whenever you’re building an AR-15, you are responsible for knowing and following all federal, state, and local laws regarding firearm ownership in your area. Rules and restrictions on firearms vary from state to state, so it’s important to be sure of them before starting your next build.
Choosing The Right Upper Length
The AR-15 has an illustrious reputation thanks to its modularity. Depending on your purpose, you can easily tailor these rifles to fit distinct roles, ranging from long-range SPR builds to ultra-compact, PDW-style carbines. As such, you need to take a step back before building one to answer this question: what is this rifle’s purpose? Is it for home/personal defense, competitive shooting, or long-range shooting or hunting? Knowing your rifle’s purpose will considerably help in narrowing down your options.
Ultimately, there’s no one upper length that’s universally better than the rest. Instead, it’s a matter of finding the right length that best fits your needs and preferences. Different upper receivers will feature different AR-15 barrel length options to better accommodate user preference. While there will be options that have more unique barrel length options, you’ll generally find that most of the popular upper receivers available have similar lengths. Below is an overview of some of the more common ones you can expect to find:
Different Upper Receiver Lengths
20-inch Uppers
One of the longest conventional options you can expect to find, 20-inch uppers are incredibly popular for their long-range performance, with them commonly being used on precision 5.56 builds, as well as hunting setups. With standard ammunition, rifles utilizing these uppers can easily hit targets upwards of 500+ yards away, but when paired with specialty ammunition like Mk. 262 or other precision 5.56 loads, their effective range can be as great as 750+ yards.
Being as long as they are, 20-inch uppers are often some of the heaviest options around and aren’t conducive to close quarters use, or other use in other fast paced applications. Essentially, with this upper length, you’re trading agility for longevity. So, if you’re building a defensive rifle that needs to be agile, this isn’t the right one for you. If you’re building a long-range 5.56 build though, we can’t recommend them enough.
18-inch Uppers
18-inch uppers are another popular option for precision builds. Uppers of this length were commonly used on Special Purpose Rifles, or SPRs, which put a focus on long-range performance with the 5.56 NATO cartridge. Despite being shorter, there isn’t much difference in terms of performance between 18- and 20-inch uppers. They both offer a similar effective range, maximizing the longevity of the AR-15. That said, 18-inch uppers are slightly lighter since they have shorter barrels.
Primarily, 18-inch uppers function best when they’re part of long-range builds like SPR clones. That said, they are short enough to offer some versatility in a general-purpose role like some of the uppers listed below. While it’s more than possible to use an 18-inch upper in a general-purpose role, keep in mind that your rifle won’t be the most maneuverable in tighter spaces.
16-Inch Uppers
16-inch uppers are one of the most common upper receiver lengths available. This upper length also falls into what most would consider to be ideal for “general-purpose” use, meaning they’re solid jack of all trades suitable for most applications. Plus, since they feature a 16-inch barrel, you can easily build a rifle without breaking any NFA restrictions.
At this length, the barrel is long enough to provide enough muzzle velocity to have an effective range of around 450 to 500+ yards away with standard 5.56 NATO ammunition, while being short enough to functionally maneuver through close quarters. Likewise, it’s common to see 16-inch uppers being used for everything from small and medium game hunting to competitive, recreational, and defensive shooting applications.
14.5-Inch Uppers
Another widely favored upper length is 14.5-inches, more specifically, a 14.5 pinned and welded upper. A 14.5-inch barrel on its own would require you to register your rifle as an SBR, but by pinning and welding the muzzle device to the barrel, it becomes permanently affixed to it, adding to its total length. So, if the muzzle device is long enough to bring the barrel’s total length up to, or longer, than 16-inches, you can treat it as you would a standard 16-inch upper.
Despite being slightly shorter than their 16-inch counterparts, 14.5 uppers still offer an incredible degree of versatility. They have an effective range of up to 450+ yards with standard 5.56 NATO ammunition, while their more compact size makes them easier to maneuver around close quarters and tight spaces, making them another solid general-purpose possibility.
Lastly, 14.5 uppers are among one of the more popular options available, so there’s a wide array to choose from. Just keep in mind that if you buy a 14.5 upper, you will not be able to replace the muzzle device like you would with a 16-inch option. Therefore, it’s imperative that you opt for an upper that already has the muzzle device you want.
13.9- and 13.7-Inch Uppers
The 13.9 and 13.7 upper receiver lengths are newer, more niche options that have had a surge in popularity in recent years. Much like 14.5 uppers, 13.9 and 13.7 uppers often come from the factory with a pinned and welded muzzle device, making them legally rifle-length. Plus, both have nearly identical levels of performance.
At their core, they offer a level of performance that’s on par with 14.5 uppers, albeit with marginally less muzzle velocity (no more than around 20 to 50 FPS). Functionally, they can still reliably hit targets upwards of 400 to 450+ yards away with no issues. Since they feature a shorter barrel, they’re often erroneously believed to be more compact than 14.5 rifles, but they have a nearly identical overall length at the end of the day. Still, they have the advantage of being lighter since they have less barrel material, making them one of the nimbler rifle options to consider.
It’s important to note that these uppers don’t have as much muzzle device variety compared to other pinned and welded uppers. Using a shorter 13.9- or 13.7-inch barrel, the muzzle device needs to be longer than usual to bring its total length up to 16 inches. Keep in mind that this is beginning to change as their popularity grows, but as of right now, your muzzle device options are somewhat limited. Still, you have some great options to pick from, such as NOX series muzzle devices from SOLGW, and the Expo Arms/Forward Controls Design extended muzzle brake.
12.5-Inch Uppers
12.5 upper receivers offer a well-rounded shooting experience. Their use of a shorter barrel makes them particularly well-suited for applications that prioritize maneuverability, such as home defense, duty, and competitive carbine courses. While compact, 12.5 AR-15 builds, on average, have an effective range of around 350+ yards, and they generally have more rail space compared to other shorter upper options like 11.5 or 10.5 uppers.
Keep in mind that with the right training, ammunition, and optic choice, it’s still possible to land hits on targets far beyond that metric—but it’s important to recognize that this isn’t what these uppers were designed for. Also, while you can theoretically pin and weld a muzzle device long enough to bring your barrel up to 16-inches, it isn’t the most practical option. So, if you want to run a 12.5 upper, you’ll have to either register your rifle as an SBR or build an AR pistol that follows ATF guidelines.
11.5-Inch Uppers
11.5 upper receivers are another incredibly popular choice for compact AR builds. These uppers are short enough to make the overall size of the AR more compact without compromising too much on ballistic performance. Overall, they perform similarly to 12.5 uppers, with them having a similar effective range.
While it may appear that there isn’t much difference between the two, it’s important to recognize that 11.5 uppers generally offer more in terms of speed since they’re shorter and a bit lighter. This makes them particularly well-suited for close-range applications.
10.5- and 10.3-Inch Uppers
The 10.5 upper receiver length is incredibly popular for compact AR-15 builds and are found on popular rifle systems like the Daniel Defense MK-18. 10.5 and 10.3 uppers are renowned for their compact profiles and close-range performance, and similar to 13.9 and 13.7-inch options, there’s hardly any difference in performance between the two. These uppers are considered by many to be the shortest practical barrel length for a 5.56 build, as they offer incredible close quarters capabilities without compromising too much on ballistic performance.
At this length, you can expect to have a maximum effective range of around 300 yards, making it best suited for close quarters use. If you’re wanting to build a compact AR-15, a 10.5- or 10.3-inch barrel are some of the shortest feasible options to consider.
Factors to Consider Before buying an Upper
Gas Tube Lengths
Your upper’s gas length plays a pivotal role in how your rifle operates. AR-15s operate off a direct-impingement gas system, meaning gas is siphoned off the barrel after firing, and redirected back towards the bolt to cycle the action. AR-15s most commonly use either a rifle-length, mid-length, carbine-length, or pistol-length gas system, which are each better suited for different barrel lengths. We go more in-depth on this topic in our article on gas tube lengths, but below is a summary of how each gas system performs:
Rifle-length gas systems are the longest traditional gas system with the gas port positioned 12 inches down the barrel, so they’re most often used on rifles that have either an 18-inch barrel or longer barrel. They extend the dwell time of the bullet, allowing the pressure to be reduced before siphoning gas to cycle the rifle. As such, they generally provide a softer recoil impulse and don’t cause a lot of wear on internal components, but they don’t offer the same level of reliability as that of shorter gas lengths, depending on which barrel length you opt for.
Mid-length gas systems bridge the gap between carbine- and rifle-length gas systems, shortening the gas port position to about 9 inches down the barrel, making them a solid option for rifles with anywhere from a 13.7- to 16-inch barrel. This option provides you with a good blend of reliability and comfort, as they generally offer a softer recoil impulse compared to carbine-length gas systems, and greater reliability than rifle length systems.
Carbine-length gas systems are widely regarded as one of the most reliable gas system options available and are great for uppers with anywhere from a 10.5- to 16-inch barrel. Carbine-length gas systems position the gas port roughly 7-inches down the barrel, so it siphons gas at a higher pressure than mid-length systems. This allows the rifle to more reliably cycle in adverse conditions, while also making it compatible with more diverse ammo types. Since it uses higher gas pressures, this gas length results in a stronger recoil impulse as well as more wear and tear.
Lastly, pistol-length gas systems aren’t a traditional AR gas-length, and they’re primarily used on shorter AR pistols and SBRs with a barrel length of 10.5-inches or shorter. This length has the shortest gas port location—just 4 inches down the barrel. As such, the gas is siphoned at extremely high pressures, making it well suited for shorter barrels or lower pressure cartridges like .300 Blackout. Still, the extreme pressure gives AR-15s with this gas system the harshest recoil impulse of the 4 gas systems mentioned, as well as the highest amount of parts wear.
Handguard Length
Just like how complete uppers can come equipped with a variety of barrel and gas length options, they can also come with a wide range of different handguard lengths.
Handguards are an important part of the rifle. By design, they protect your hand from the heat of your barrel and gas block after firing, and they’re also serve as the mounting hub for most accessories like sling mounts, grips, lights, and laser units. As such, having the right length of handguard is key for your next build. Ideally, it should be long enough to cover and protect the gas block, without extending past the muzzle.
This length range offers the most in terms of component protection and accessory balance. Any shorter, and the gas block would be exposed—and having it extend past the muzzle would pose a risk to your hands and any gear you have mounted at the front end of your rifle.
Accessories
As we mentioned above, one of the key advantages of buying a complete upper is the variety of options available. There’s a wide range of upper models to choose from, and they each come equipped with different rail systems, muzzle devices, and other accoutrements. So, in addition to the barrel length and gas system, you’ll want to make sure that your upper already has the rail system and muzzle device you want (especially if it’s a pin and welded upper).
Uppers can be quite costly, and while you can always swap out one’s individual components, the cost can add up quickly. So, we recommend finding an upper receiver option that already has most, if not all, of the parts and components you want for your build. The vast assortment of models to choose from means that there’s something for everyone, so you’re likely to find your ideal upper in time.
Conclusion
AR-15s will always stand out for their adaptability. As one of the most modular platforms ever created, they can easily be configured to fit all needs, ranging from smaller compact rifles for home-defense, to mid-sized general-purpose builds and even long-range precision setups. With such adaptability comes options, and there’s no shortage of upper receivers to choose from when building yours.
When you’re selecting your next upper, remember to take your AR’s intended use into consideration. Like we said above, there are a plethora of upper receivers to choose from, each with different barrel lengths that suit different applications. This makes the best upper receiver option the one that meets your needs and preferences best. So long as the one you’ve chosen checks off all your boxes, you can rest assured knowing you made the right call.
