In this piece, we’re going to take a pass at addressing the question: what’s the shortest effective barrel length for an AR15? To do so, we’ll start with a little bit of general ballistic science to get some basic concepts under our feet. From there, some specific commentary on the operating mechanism of the AR platform, specifically the gas system, is important.
But first, let’s define effective. Here, we mean an effective barrel length as one that can be used in an AR with which you can reasonably expect to engage human-sized targets out to about 300 yards with some skill involved.
Ballistic Science 101
When a bullet is fired, the burning powder builds pressure behind the bullet, pushing it forward through the barrel, where the bullet roughly seals against the rifling, accelerating as it travels down the barrel.
In general terms, greater barrel length means more velocity, which, in turn, means less bullet drop and greater accuracy. Of course, there’s a limit to this: a six-foot barrel would not net you many gains in velocity, but it would make the rifle extremely unwieldy, something like an old-timey Punt gun, which was a bow-mounted shotgun people used to hunt ducks.
AR15 Gas Systems
Regardless of barrel length, the AR15 is a direct gas impingement gun, requiring a gas tube that runs parallel to this barrel. To have the gun cycle as a semi-automatic firearm rather than a bolt action one, you’ll need to have a function gas system. These days, gas systems come in several more or less standard lengths.
Pistol length gas systems are the shortest, and they’re about 4” long. A carbine length system is 7” long. Further, a midsize gas system is 9” long and the longest, rifle systems are 12” long. These, as you’ll see soon, correspond to several barrel lengths, and with accompanying pros and cons.
Six Inch Barrels
About the shortest barrel that you can hope to have normal function in an AR is a 6” barrel. These will have substantial accuracy problems because of a short sight radius as well as the fact that the bullet will not get up to normal velocity, resulting in a bullet drop. These will function with a pistol-length gas system and might work for extremely close quarters work, but be careful as you’ll have your hands awfully close to the muzzle at all times.
Ten To Twelve Inch Barrels
With a little bit longer barrel comes a little more velocity: with a carbine gas system, these short ARs can gain some serious accuracy over the extremely short models, but you’ll still notice that bullet drop will start sooner than you’d like.
This is a common length of AR for folks in special forces who know that their mission will take them indoors, as this is a compact weapon that still can reach out to about 200 yards with a little bit of practice. Overall, these are the first reasonable option we think is out there if you want accuracy past room-clearing distances.
Thirteen to Sixteen Inch Barrels
This is, in some regards, the Goldilocks zone for AR barrel lengths, as we’ll develop in the next section. Paired with a Carbine length gas system, rifles with barrels of this length give up very little in terms of velocity when compared to longer barrels but are a little more compact than longer weapons. Additionally, the carbine gas system, balanced with an appropriate buffer spring and weight, are exceptionally soft recoiling weapons, which makes follow-up shots a lot easier.
Twenty Inch Barrels
About the longest barrel you’re likely to see on an AR platform rifle is 20.” These give the greatest possible velocity out of standard 5.56mm ammunition, without being too bulky for people to use in adaptable mission settings. Barrels of this length are less common now but are a good choice for folks who want to stretch the legs of the platform. With a rifle-length gas system, they’re also exceptionally reliable weapons as well, as this is basically the design as it was first developed in the 1950s.
So, What’s the Best Barrel Length?
As with most things in the firearms world, the answer is: what’s your purpose?
If you want the best possible ballistics, then a 20” barrel with a rifle-length gas system is the way to go. Even though these are long and a little heavier than what’s common these days, most accuracy-focused setups will have longer barrels to get additional velocity and a longer sight radius for iron sights. The M16A1, as issued in Vietnam, came with a barrel of this length and it performed well for troops for years.
Of course, some people don’t want the best ballistics, and they are looking for a handy gun to use in home defense. To that end, a Short-Barreled-Rifle might well be the most effective: a 10” barrel with a pistol length gas system can be highly effective in close quarters and they’re super handy. If it were up to us, we’d also put a suppressor on it to tame the unholy muzzle blast that comes from all of the unburnt powder and unused energy from short barrels.
For most folks, something in between works well. Enter the M4, the standard service rifle in the hands of several million people in the armed forces all over the world. With a 14.5” barrel and a carbine length gas system, the M4 can reach out to about 300 yards with no issue but is also handy enough to be pressed into room-clearing duty in, say, Fallujah. This is what we go with most of the time on our personal rifles.
With all of that said, and government paperwork aside, you don’t really have to choose. It’s entirely possible to have a single lower receiver, a few buffer springs, and a set of uppers that you can change out in seconds depending on your mission requirement. That’s one of the brilliant things about the AR 15 design, and we personally own several different barrel lengths for different tasks.