AR Barrels Explained: From Basics to Bores to Ballistics

An AR barrel butts up against the upper receiver to hold a chambered cartridge in place before firing, then usher the bullet out the muzzle and toward the target once fired.

Most barrels are made of steel (at times paired with carbon fiber or other durable material) and rifled (a set of grooves in the bore) to provide spin on a bullet as it passes down the length of the barrel.

Barrels are organized and categorized by their various attributes: length, weight, composition, surface treatment, gas system, chamber size/caliber designation, and rifle twist rate. All of these inputs have performance implications, which can make selecting a barrel a real challenge.

Lucky for you there are a few common combinations that are almost standard these days, with the distribution of barrels largely falling along a power law curve, e.g. for every odd duck barrel in the wild there are 10 AR-15s paired with an M4/A2 profile barrel in a medium-weight.

Why?

In the classic case of “if fit ain’t broke” these basic profiles just work — and well enough that rationalizing another barrel shape really requires a pretty serious edge case.

The Implications of Lengths & Weights

While the federal law doesn’t pay much attention to barrels over 16-inches (no matter how long they may be) a rifle is indeed subject to the NFA if it has “a barrel or barrels of less than 16 inches in length“.

Under 16 inches and you’ve meandered into Short Barreled Rifle (SBR) territory if you plan on running a stock (AR pistols require a pistol brace). Unless you’ve got your tax stamp in hand this is best avoided. 

A good general policy is to leave your carbine-length barrel a carbine-length barrel. Any shorter and you’re playing with fire (bother literally and figuratively).

Stepping out beyond the 16-inch carbine barrel, the next lengths you’re going to encounter are 18 and 20 inches. The 20-inch length is the original AR-15 rifle length and the one for which the 5.56 round is optimized. It’s also the length many turn to when creating or shopping for a full-sized rifle.

Of course, there a number of barrel lengths well under 16 inches — with 14.5, 11.5, 10.5 and 7.5 inches in .223/5.56 fairly common, and .300 BLK barrels adding 8 and 9 inches to the mix. Whether you use these shorter barrels for the creation of an AR pistol or venture into SBR territory is indeed your choice. 

One caveat here: you can run a sub-16-inch barrel and avoid NFA paperwork if you permanently attach a suppressor or other muzzle device to a 14.5-inch barrel provided that attachment pushes the total length over 16-inches. More on this below.

Given internal bore is the same from barrel to barrel, a barrel’s weight is therefore a function of the external diameter. A barrel blank (essentially a rifled steel cylinder with all material intact) is going to add upwards of 6 pounds to your rifle — and run more than an inch in diameter, which nobody is going to want to wrap a handguard around. 

The act of removing that unnecessary material by turning the barrel on a lathe is called “profiling,” which hones the former cylinder into a usable rifle barrel.

On Profiles

The barrel profile is another area of give and take — heavy barrels last longer, take longer to heat up, and are easier to shoot accurately but are, well, heavy. A lighter barrel carries and points much easier, but without the mass up-front, you’ll experience more felt recoil.

“The Right Barrel” therefore depends on your needs. If you’re venturing into the woods a lot or planning a home defense build, you would do well to select a lighter barrel, while folks who tend to spend time at the range (or shoot off the back porch) can run a heavier barrel without incurring any of the downsides.

Barrels fall into three general “profile” types– lightweight, government, and heavy. Ironically, the hallmark of the early AR and subsequent AR-15 was a very thin barrel as the gun was developed by ArmaLite, a subsidiary of a big player in the aviation industry, one in which every ounce mattered.

Lightweight barrels still have a place these days as the steel barrel– particularly on a firearm with aluminum receivers and plastic furniture– accounts for a lot of the total weight. Therefore, the lighter the barrel weight, the lighter the gun’s overall weight.

Today such traditional “pencil” barrels would be seen as lightweight as the Army, once the AR-15 was adopted and fielded in Vietnam in the 1960s, found that heavier barrels were more desirable to their needs.

This is because a thin barrel, although lighter for the soldier that must carry it, also heats rapidly in use, a factor that will shift the point of aim/impact after even a modest number of rounds without cooling and contribute to barrel whip. Also, by dint of recruitment realities, the Army is dealing with young, overzealous users who will likely fire round after round (after round after round…), so a heavier barrel will ensure they get as much life out of their rifles as possible. This may or may not fit your usage profile, so adjust your barrel selection accordingly.

Today, the slightly beefier barrel is referred to as having a “government profile” as it has more steel than a lightweight pencil barrel but falls short of a target barrel.

Regarding heavy profile barrels for ARs, Colt’s original H-BAR was designed for match target competitions and was introduced in 1986. Taking this to an extreme are full bull barrels such as Criterion’s 24-inch model, meant for precision varmint use and similar applications.

Cutting down on weight, heavy barrels that utilize carbon fiber sleeves or extensive fluting keep rigidity and accuracy at a distance while trimming the ounces.

However, such options come with a corresponding expense. I cover this more in the section on barrel contouring.

Materials & Stiffness

Although stiff as can be in one’s hand, a barrel moves around (or “whips”) when fired. This flexing is a byproduct of the bullet speeding down the bore with the forces of spin applied via the rifling and the upward motion of the muzzle brought on by muzzle rise.

As with any variable in shooting, less is more. Stable barrels are more accurate barrels, and heavy barrels — owing to their additional material and stiffness — move less, all things being equal. In fact, shorter barrels tend to be stiffer than their longer equivalents because stiffness is influenced by length and diameter.

Alloy Steel

Prior to legions of AR fanatics pondering over the composition of their barrel steel, the standard was an early high-alloy steel known as 4140, or Ordnance Steel. The Society of Automotive Engineers (or SAE) uses a simple four-character system to help you understand a steel’s composition. The first number identifies the class—carbon, nickel, chromium etc. The next three numbers detail the quantity of additional material in the final product.

Taking a page out of red-faced online forum combatants, let’s look at 4140 and 4150 steel.

• 4140 indicates the barrel contains .40% carbon content
• 4150 indicates the barrel steel has a carbon content near .50%

That additional fraction of carbon adds a ton of stiffness and hardness to the steel, which both makes it more challenging to form but also more accurate and durable. Will you notice the difference? YMMV.

416 Stainless Steel

You’d be forgiven for conjuring images of cutlery with the mention of stainless steel, but stainless barrels are indeed different from the material in your friendly neighborhood butter knife. In fact, stainless barrels have historically been more exacting and accurate than their alloy steel counterparts, mostly due to the craftsmanship involved.

A stainless barrel is often the go-to for serious competition shooters because fractions of an inch matter, and they’re willing to pay for the exacting standards expensive stainless steel manufacturers can provide.

While neither are truly “stain-free”, 416 or 416R — the most common stainless steels used for barrels — will rust more slowly than alloy steels. As compared to steel, 416 has less than half the carbon (around 0.15% vs. 0.40% in the case of 4140 or .50% with 4150), but between 12% and 14% (!) chromium and around 1% manganese, so it’s plenty hard while pairing ease of machining with durability.

416R offers even more machinability so premium barrel makers love it because they can polish the bore (a process called “lapping”) that adds accuracy and reduces fouling deposits. The main difference for Average Joe’s? Price. 

Carbon Fiber

No, you find a bore composed of carbon fiber, but it is often used as a wrap around the barrel. This addition of exterior carbon fiber requires a slightly slimmer barrel diameter, but the carbon fiber adds stiffness and heat dissipation while shedding the weight of the steel.

The downside? Sticker shock. Carbon fiber barrels can often cost more than a complete rifle.

Barrel Linings

In terms of barrel linings typically seen on AR rifles — and for that matter, most rifles — the two most encountered are nitriding and chrome-lined, both of which make the barrel harder and more durable than steel alone.

Nitriding/Meloniting/Armorniting are gas nitrocarburizing processes that change the surface properties of the barrel itself– inside and out– while chrome lining adds a layer of metallic chromium material to the surface of the barrel’s interior and chamber.

Chrome barrel lining dates back to at least World War II, being common originally on military rifles due to its heat resistance properties, Nitriding et al. is a more modern process and is easier (and cheaper) to apply as well as having the side effect of protecting the entire barrel, including the exterior.

With that, don’t confuse chrome-moly or chrome-moly-vanadium (CMV) barrels– which are made from alloy steel that includes mixes of chromium-molybdenum and/or vanadium– with a carbon steel barrel that has been chrome lined.

Rifle barrels all begin life the same way– as a piece of high-quality steel bar stock, be they Chromoly, CMV, carbon, or stainless-steel varieties. What comes next is shaping and milling down to produce the desired-sized barrel blank, which is then processed to produce the internal rifling through a center-cut bore.

This is accomplished by either cold-hammer forging (CHF), cut rifling, or button rifling. 

• Cold-hammer Forging: without getting too much into the weeds, CHF barrels are made by forcing a rifling-shaped mandrel through the center of the blank while at room temperature, then stress-relieving at a higher heat afterward. This produces a strong, uniform rifling that is accurate and will outlast most other rifling types, making it preferred for rifles intended for hard or extended use, such as for military contracts. It is a process that also scalable and is commonly seen in use with barrel makers like FN, who crank out thousands of barrels from raw stock every month.
• Cut rifling is a time-honored but also time-consuming process in which a bored barrel blank is inserted into a rifling machine then “cut” out thousands of an inch on each pass. It takes a while to make such barrels, which makes them more expensive. Still, it is worth it as barrels that utilize cut rifling are typically capable of delivering precision results downrange. For this reason, many custom gunsmiths favor cut rifling for their barrels.
• Button rifling gets its name from the process in which a tungsten carbide “button” is attached to a steel rod that rotates precisely to produce the twist rate as it is pulled through the bore of a barrel blank by a hydraulic cylinder, thus producing the grooves inside a finished rifle barrel.

Like the CHF process, button rifling is a cold-forming process that requires the stress-relieving of the steel afterward, followed by lapping to make the rifling uniform after that step. As CHF tends to be more efficient, especially with the right machines, and cut rifling is more accurate, button rifling has largely been replaced but is still seen in some legacy barrels. 

While introduced with a standard barrel, AR-15s have evolved to a host of other barrel contours to match the needs and desires of the user.

Starting with the M16A2 in the 1980s, this morphed into the government– and later M4 and SOCOM– profiles which started beefily then tapered to a thin barrel under the weapon’s handguards, had a cutout for M203 40mm grenade launchers to attach, and ended with a beefy muzzle to give the sight post/gas port a stable base.

This medium weight contouring helped trim unneeded ounces from the steel while meeting the utilitarian checklist intended for the gun. Heavy target, partial bull, and full bull barrels, particularly when mated with the appropriate handguards, have a more uniform straight wall profile.

When going for a lighter contour, such barrels can be turned down through exterior fluting, dimpling, or the like, which maintains interior bore diameter and barrel strength while removing excess weight.

As mentioned above, avoiding NFA paperwork with an SBR running a 14.5 inch barrel requires the permanent attachment of a muzzle device (flash hider, suppressor, or simple metal tube) that pushes the total barrel length longer than 16 inches.

Just be mindful of how the ATF measures barrel length and what they mean by “permanent”. Otherwise, you may come up, uh, short.

Specifically: “The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at thefurthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, andmeasured.”

The blind pin weld is far and away the most popular, as it provides a clear indication of how the attachment was welded to the barrel, avoids excess heat transfer to the barrel itself, and uses a small pin head which keeps the attachment firmly in place. This approach is popular enough that many 14.5-inch barrels are designed with this option in mind.

It’s not all upside though, as a welded extension prevents you from changing out almost anything that requires barrel removal — that includes your gas block, handguards, and front sight block.

Oddly enough, despite spilling 4,000 words of digital ink on the topic of barrels, I could have probably gone on for another 4,000, but I’ll save the nuances of aluminum grades, treatments, and barrel-specific recommendations for another diatribe.

If you want a great AR barrel, you could go way off the beaten track and spend a lot of money on something bespoke, polished, and sure the be the envy of your peers, or you could just run an M4 government and call it a day.

The well is deep, but at times the water cold, so don’t go chasing waterfalls friends.  

• Shooting Software Barrel Whip Dynamics
• Rifle Shooter Mag Guide to Gun Metal
• Wikipedia 41XX Steel