AR-15 Barrel Profile Testing: Heat, Size, & Why It Matters

Big-game hunting guide Dan Harrison tells all his clients one universal rule when checking zero: “Three shots. No more.” The reason is that the hunters he guides to trophy bulls and bucks will consistently take that crucial shot with a cold barrel. Shot placement is pivotal. Their zero should reflect such a cold-barrel scenario.

Still, come hunting season, I’ve sat at the range beside deer hunters popping off rounds, spitting brass across the bench like bullets, powder, and primers grow on trees — all the while chasing that shifting zero.

Generally speaking, hunting rifle barrels are light and thin. They’re meant to be carried long distances, in the backcountry chasing bugles or over Alaskan ridgelines, glassing for Bullwinkle. 

In the Midwest — whitetail territory — the main criteria are not creating a cramp in grandpa’s leg when it’s time for him to lean back against a tree and babysit the back of his eyelids.

But AR platforms, depending on potential intended use, are different. It’s not the first few shots that should dictate one’s zero, nor the 15th or 50th, necessarily. “Perceived potential applications” is the key here. More candidly, in nomenclature to which most can relate: SHTF.

While they’re a lot of moving parts (literally) in what equates to the perfect rifle setup for your SHTF rig, we need to start at the foundation, and that’s with barrels.

Heat causes metal to expand. More rounds fired — which creates friction from the round itself traveling down the barrel coupled with the gases from a small-yet-contained explosion — result in a barrel getting hotter with each trigger pull.

A very slightly expanded barrel means less friction on a bullet as it travels toward the muzzle, resulting in higher velocities but also less contact between barrel rifling and the projectile. A shift in velocity will affect point of impact (POI), while a looser bore and less rigid barrel will open up shot groupings.

Heavier barrels take longer to heat up, but they also cool down slower. So, while the heavier the barrel, the more time it’ll take to throw rounds and move that POI, once the thermal-dynamics threshold is reached, a shooter can expect to operate in that altered state for a sustained period of time. Whereas a shooter shouldering a pencil barrel, for example, may experience anomalies sooner but for a shorter time period.

A barrel profile, simply put, is the thickness of a barrel and the shape from throat to muzzle. Standard profiles exist, such as the M4 or SOCOM barrel profiles used by the military, while proprietary barrels are also available, such as the Ballistic Advantage Hanson barrel. These profiles are often a mix of a couple of standard barrels, uniquely engineered with the goal of a more-balanced barrel for intended applications.

Lightweight and accurate, the pencil barrel was originally carried in Vietnam via the M16A1 during long marches when extra ounces were a significant factor.

Concerns regarding warping of pencil barrels during sustained fire led to the creation of this barrel. The military wanted to keep the ability to mount the M203 grenade launcher without changing its mount, leading to a weird profile that no one in their right mind would ever spec in the commercial realm. Thicker than the pencil barrel, more weight rests toward the muzzle, potentially making it forward-heavy unless running more weight toward the rear of the rifle.

Not light, not heavy. This barrel is the middle ground. Suitable for higher rates of fire and/or sustained strings of fire, this barrel results in less whip but is also less affected by heat from gas over time.

Heavy, thicker. Results in slower times between target transitions; but the extra weight can become a factor — other than split times — when mobile over an extended period of time.

The heaviest of barrels. For an extended period of time, best supported by some form of sticks and not the body of a human. But it’ll stand up to heat and maintain its accuracy for longer versus other barrels, as well as endure a higher round count before needing replacing.

The concept started off fairly simple: Testing different profiles of 16-inch barrels by recording an initial five-shot group, dumping 30 rounds, checking temperature of the barrel nut and throat of the barrel via an infrared thermometer, then shooting another five-shot group and measuring shift in both POI and shot grouping — all the while measuring velocity before and after emptying a magazine in short succession.

“Rabbit holes for every rabbit hole” was the conclusion the more barrels we pointed downrange. As much as this entire test would ideally be 1:1, variables persisted. For example, while all barrels varied from either a 1:7 or 1:8 twist rate, every precision shooter understands every barrel is unique and will prefer one load, one bullet, over all else. We alternated between factory-loaded Remington 69- and 77-grain Premier Match when measuring groups and POI. Then, add the topic of gas lengths and dwell times — and then how a BCG and buffer spring will contribute there.

You, as a reader, are welcome to review summarized data here. Make your own conclusions, but the following truths became evident in regard to the SHTF topic.

Not 16-Inch Rifle Barrel Testing

16-Inch Rifle Barrel Testing

*Cold-hammer-forged, chrome-lined barrel
^Long-stroke piston system; others are direct-impingement
+Shot 77-grain Remington Premier Match; others shot 69-grain

It’s likely not the first 10 or even 20 shots that should determine your zero. Get to know your rifle — get to know what she likes, what she loves, and twist her finer parts just right as needed. After several rounds in quick succession, your group will open up, but it will probably also shift — in some cases enough to make a 200 yard head shot an iffy proposition. Without actually testing your own rifle, you won’t know where to hold to make that shot. Some other points to consider:

Ounces are pounds; pounds are pain.

As already discussed, heavier barrels will probably maintain a zero for longer. But they’re less comfortable for a few reasons.

Punching paper off a bipod or casually shouldering at an indoor range is far different than carrying your rifle — or running with your rifle — for miles. Once you’re tired, perhaps with a heart rate exceeding 180 bpm, you’ll feel the weight. Heavier barrels are accurate for longer. Heavier barrels are heavier.

Then, there’s the subject of target transitions. Some professional competitive pistol shooters won’t run a light because the 1.4 ounces on their dustcover slows down their splits between targets.

Heat doesn’t stop just because you’re done shooting.

What isn’t shown in the charts is the increase in surface temperature of a barrel left unfired after 30 rounds. It continues to get hotter, upward of 10 degrees F hotter over several minutes. From there, depending on surrounding conditions — and the barrel itself — it could take an hour or longer to return to its starting temperature.

A suppressor will increase this effect. During testing, for additional insight we ran SilencerCo’s Velos 5.56 LBP. During a mag dump, the can acted as a heatsink, but trapped gases seemed to slowly work their way down the barrel, increasing the barrel’s temp over time versus running unsuppressed. Perhaps this is an argument for always running mount-ready muzzle devices and carrying a thick glove. Or not. For a brief few minutes though, the throat of the rifle barrel, temp-wise, was largely unaffected while the suppressor smoked like a branding iron.

An argument against piston-system guns is extra weight (ounces, but varies by length), possibly extra recoil due to more reciprocating mass, and, at farther distances, the drop in accuracy due to reciprocating mass affecting barrel harmonics.

But a piston-driven gun bleeds off more gas further from the chamber, resulting in less barrel heat affecting accuracy, as well as a cleaner chamber over an extended period of time. At 16 inches, during this testing, the Primary Weapons Systems MK116 Mod 2-M outperformed every other rifle, significantly so, but I wasn’t pushing her 400 to 500 yards, and I was shooting from a bench where my arms received ample rest between shots.

Without corrective lenses, I’m legally blind, so I’ve come to appreciate good glass. Sustained fire results in mirage, as heat rising from a hot barrel (or hot, hot suppressor) distorts air and creates a shimmering effect. At this point, it helps to lower magnification, but if running less-than-quality glass, clarity suffers. Cheap glass functions decently at high magnification and far less so at lower settings.

For the majority of this testing, we swapped Leupold’s Mark 5HD 2-10×30 M1C3 FFP PR1-MOA between rifles. This glass made a major difference during hot-barrel testing, especially when shooting from behind a hot can, as we could dial back and still feel confident in shot placement in relation to overall sight picture.

Which begs the question: Do you run a quality LPVO, perhaps some canted sights; or a HWS, magnifier, set on a riser? Or perhaps your eyes are so good you can rock irons and hammer steel, no issue, from 400 yards. 

Reach out to me on Instagram (@WildGameJack) with any questions or comments.