Whether you’re planning to build a custom AR-style rifle at home or just looking at one in a shop and wondering how to categorize it based on length, it’s important to keep in mind that there is more than one way to do so.
Some users categorize length by the total length of the upper, others by the gas system or the barrel.
Here’s what you need to know about AR upper, barrel, and gas system length, which you can apply to what you know about pistol-length and full-sized 20-inch AR upper receivers.
Pistol-Length AR Uppers
It’s important to know whether what you’re looking at is categorized (legally) as a rifle or a pistol, because while pistols can (obviously) have short barrels, a rifle with a barrel length of less than 16” would fall under the NFA and possession of such a “short-barreled rifle” or SBR is severely regulated.
That said, the shortest AR uppers (and gas systems and barrels) belong to AR pistols. AR pistols, which are commonly chambered in .223 Remington or 5.56 NATO, just like rifles, usually have very short gas systems of around 4.5 inches and barrel lengths around 7.5 inches.
The short gas system length produces a fast cyclic rate and a heavy cycle, which usually results in fairly heavy recoil, making AR pistols somewhat difficult to control with one hand. This makes devices like AR pistol braces popular because they add stability, as well as muzzle brakes, which cut felt recoil.
AR Carbine Uppers
Next we have carbine-length AR uppers, which usually have gas tubes of about 7.5 inches, and barrels between 10 and 18 inches.
This compact size and lighter design makes AR carbines highly popular for home defense, as they are maneuverable, even in confined spaces, and tend to feature similar ergonomics, handling, and expandability to much larger, heavier rifles.
Mid-Size AR Uppers
Mid-length AR uppers occupy a gray area between what some might call carbine-length rifles and others full sized platforms.
However, most mid-size ARs are just a little bit longer than carbine-length platforms, with gas tubes in the area of 9.5 inches and 14 to 20 inch barrels. These platforms pair some of the advantages of full-size rifles with the lighter nature and more compact design of their smaller, carbine-style cousins.
Rifle Length AR Uppers (20 Inch AR Uppers and Bigger)
All the way on the opposite end of the spectrum from AR pistols we have full-sized AR-15 rifles, which have the longest gas tubes, barrels, and overall upper lengths of all.
Rifle length gas systems are the longest of all, at 13 inches, and full-sized AR platforms have long barrels, usually 20 inches or longer, with lengths to 24 inches not uncommon. So, if you’re building a rifle with a 20 inch AR upper, it is probably a full-sized model (definitely if the barrel itself is 20 inches).
Full-sized rifles are the longest, largest, and heaviest AR platforms, which makes them a little less suitable for use in cramped quarters. However, they are commonly used for long-distance shooting as they are more stable, and the longer barrel tends to produce higher muzzle velocities and greater overall accuracy.
Gas System Length
Another way to break these details down is by looking at the parts, piecemeal, and one that has a pronounced impact on the overall performance of the platform is the gas system.
The gas tube, which conveys gas from the barrel and siphons it off to the bolt carrier group to cycle the action, rides on top of the barrel and under the handguard. With the handguard removed and the barrel and gas tube exposed, you can often identify the side of the rifle just by the tube itself.
Length of the gas tube is a matter of performance, too. If the gas system is too short it will siphon off too much gas, which is much harder on the gas system and the gas rings of the BCG. Parts will wear out faster if you use a shorter gas tube than you need to.
Conversely, if the gas tube is too long, it may not deliver enough gas to cycle the action, because there will be too much volume for the gas to expand and not enough gas. This can cause a whole variety of problems, not the least of which are jams.
There are four standard gas tube lengths. These are:
- Pistol length gas tubes at 4.5”
- Carbine length gas tubes at about 7.5”
- Mid-length gas tubes at 9.5”
- And full-size rifle length gas tubes at 13”
Barrel length is another consideration that determines the performance of the rifle with respect to handling, muzzle velocity, power, and accuracy.
AR-15 barrels commonly range in length from 7.5 inches to 24 inches, with 16, 18, and 20 inch barrels among the most popular.
Barrel length is not the only determinant of barrel performance (rate of twist is, too, as well as what the barrel is made of, such as chrome-moly-vanadium alloy) but in general, a longer barrel will be heavier (but more stable) and a little harder to maneuver. It will also produce higher muzzle velocities and offer greater accuracy overall.
The shorter the barrel length, the better the rifle will handle, but muzzle velocity and accuracy will usually take a bit of a hit - but again, this doesn’t account for rate of twist.
Overall AR Length
Another way to categorize your AR-style rifle is to measure the upper length itself. While this isn’t as helpful because it doesn’t really impact performance, handling, or power the way the gas system and barrel will, it can help you choose accessories like cases, because to get a properly sized case you would need not only to know the upper length but the overall length.
Shop 7.5, 10.5, and 20-Inch AR Uppers
Getting ready to get started on an AR home build? We sell stripped uppers, complete assembled uppers, and entire AR build kits, along with mil-spec parts, stocks, grips, barrels, buffer tubes, muzzle devices like brakes and A2 flash hiders, gas blocks and tubes, bolt carrier groups, charging handles, trigger groups, and everything else you need to complete a functioning rifle.
Check out our collection and get in touch with us if you have any questions before you start.