Why 11.5 is the best AR barrel length for most shooters

Let me start by saying that shooting 55gr .223 out of a 20-inch pencil barrel with a rifle-length gas system, as Eugene Stoner, Bob Fremont, and Jim Sullivan intended, is a very enjoyable firearms experience. That said, since the AR-15 was introduced, shooters and engineers have been trying to figure out just how short an AR’s barrel length can be.

The AR’s internal pistol design bleeds gas from the barrel and uses it to cycle the bolt carrier group. With a shorter barrel and gas system, the diminished distance between the gas port and the muzzle can increase the bolt speed. Known as dwell time, this can cause a short AR to cycle violently and more quickly degrade its parts.

During the Vietnam War, Colt engineers cut down the M16’s 20-inch barrel to 10 inches and made the Colt Commando or Model 609. Designated the XM177E1 by the Army, the carbine was lighter and more compact than the M16, but also extremely loud. A moderator muzzle device with an expansion chamber was added to tame the extreme muzzle flash and control the blast from each shot.

But the short barrel also reduced bullet velocity and the timing of the modified gas system caused malfunctions like failure to extract empty cases.

Improving on the 609, Colt came out with the 629, designated the XM177E2. With an 11.5-inch barrel, the E2 wasn’t nearly as blasty as its predecessor. However, it still suffered from range, accuracy, and reliability issues. The Air Force also adopted Colt Carbines (CAR-15) as the GAU-5 family and issued them from the Vietnam era through the Global War on Terror.

In 1982, the U.S. government asked Colt to develop a carbine version of its latest M16A2 service rifle. Building on the previous XM177s, Colt came up with the M4 and its 14.5-inch barrel. Originally developed for rear echelon troops, the M4 suffered from reliability issues in the hands of SOCOM operators who put extremely high round counts through them.

A heavier SOCOM barrel helped to correct these issues and was formally adopted in the M4A1.

Soldiers firing the Mk-18-Mod-0. (Photo by Pierre-Etienne Courtejoie/U.S. Army)

The M16A2 and M4 also introduced the 1:7 barrel twist. Faster than the old 1:12, it could stabilize heavier rounds like M855 and Mk262 for greater accuracy.

Taking the M4 a step further, Naval Surface Warfare Center Crane cut the 14.5-inch barrel down to 10.3 inches. With the muzzle just in front of the front sight gas block, the even shorter Mk 18 became a favorite of Navy SEALs and Green Berets fighting in the streets and buildings of Iraq. It also made Visit, Board, Search, and Seizure operations easier for sailors LARPing as SEALs.

However, the extremely short barrel brought on the same issues of increased wear and faster cyclic rates. This was further aggravated if a Mk 18 was fitted with a suppressor, which increased backpressure in the gas system.

With the AR-15 platform improved with barrel and technology, the solution to the hard-cycling Mk 18 was revived from the Vietnam-era XM177E2: an 11.5-inch barrel. That extra bit of barrel increases the dwell time in a carbine-length gas system, easing up on the gas cycling back into the firearm while adding just a bit of overall length.

It’s just over an inch, but an inch matters.

Special Forces Soldier with an URG-I-Bus barrel on his rifle. (Photo by Staff Sgt. Joshua Taeckens/U.S. Army)

With the widespread adoption of AR-15 pistols, 11.5-inch ARs became more commonplace in the civilian marketplace since they didn’t need a $200 tax stamp and registration like a short-barreled rifle (SBR). Military adoption of the barrel length came with the Geissele Upper Receiver Group-Improved (URG-I) in Army Special Operations Command.

The URG-I comes in both 14.5 and 11.5-inch barrel lengths so that the operator can swap upper receivers on the same lower receiver, as the mission dictates. Despite the slightly longer barrel, the switch from a railed to M-LOK handguard keeps the 11.5-inch URG-I at about the same weight as the 10.3-inch Daniel Defense Mk 18 Mod 1 upper receiver.

While the URG-I is only fielded within USASOC, the rest of SOCOM is getting on the 11.5 train. Sons of Liberty Gun Works (SOLGW) won the SOCOM Combat Assault Rifle contract with their 11.5-inch MK1. Looking to the future, SOCOM’s new Hypervelocity Improved Combat Assault Rifle program calls for a rifle with a barrel length between 11-12 inches.

Another benefit of an 11.5-inch barrel length comes with the addition of a suppressor. As long as it’s properly gassed and tuned, a suppressed 11.5 ends up being about the same overall length as a 14.5. Of course, gas and tuning is less of an issue with a low backpressure can like the HUXWRX Flow 556k “Black Magic” suppressor specifically named in the HICAR contract.

Related: How the AR-18 influenced a generation of service rifles

A Special Forces Soldier firing his rifle with a suppressed URG-I barrel. (Photo by Pfc. Anthony Barreras/U.S. Army)

That’s great for the military, especially considering that other weapon systems like machine guns, drones, mortars, and air support can be employed in addition to a rifle. Looking at domestic use, again, it comes down to mission. Law enforcement might actually have use for an insanely short 7.5-inch barrel for SWAT Team room clearing, but require a longer barrel if their department issues a certain type of ammo.

In an interview with TFB TV, FBI Ballistic Research Facility Senior Ballistician Robert Martin stressed that purpose and projectile will dictate the ideal barrel length. That said, Martin noted that “…11 and a half inch gave [the FBI] everything that the 14 and a half was giving us.”

With the tax being dropped to $0, the financial burden for a civilian to buy or make an SBR has been removed. The same can be said of suppressors which, again, pair very nicely with an 11.5 inch barrel length. Moreover, a 2026 court ruling means that firearms purchased from Palmetto State Armory by Gun Owners of America members can be converted into SBRs without taxation or registration (as state and local laws permit).

I’m not a lawyer and this isn’t legal advice, so there’s also the AR pistol and brace route for shooters wanting an 11.5-inch barrel.

In addition to Geissele’s URG-I, Daniel Defense offers an 11.5-inch barreled upper receiver with their RIS III rail, an evolution of the 10.3-inch barreled upper receiver and RIS II rail from the Mk 18 Mod 1. SOLGW also offers a civilian version of the MK1 for customers looking for an 11.5 with a military connection. Otherwise, most AR manufacturers offer some variant with an 11.5-inch barrel. Firearm and ammunition technology might change in the future, but for now, 11.5 is the sweet spot.

Editor’s Note: This article by Miguel Ortiz was originally published by WeAreTheMighty.com.

Feature Image: Soldiers during a room clearing exercise. (U.S. Army)

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