Barrel Profile

Almost all AR-15 barrels begin their lives as a 0.980″ tempered steel rod. The rod is then drilled, rifled, heat treated, profiled, and finished into the final product.

The barrel profile determines three things:

Barrel Weight: The thicker the profile, the more metal, and the heavier the barrel. The contribution of the barrel to the overall firearm weight may be a consideration when selecting a barrel profile. Practically speaking, the more you need to move around with the gun, the less you want it to weigh.
Axial Stiffness: The thicker the profile, the harder it is to flex the barrel. This axial stiffness reduces the amplitude of the barrel harmonics (i.e. less barrel whip). By taming the harmonic oscillation of the barrel, you reduce the movement of the muzzle. While you can tune the cartridge to time the exit of the bullet from the muzzle such that the muzzle is in a neutral position, reducing the amplitude of the barrel whip greatly reduces the variation seen under varied conditions and loads.
Thermal Stability/Mass: Assuming a series of barrels made from the same material, the less material, the faster it will heat up and cool down. As the barrel heats up, the physical properties of the metal change. The bore expands as the temperature rises, which can affect gas pressure and bullet velocity. A hot barrel will vibrate differently, which will affect harmonics. Sustained fire can lead to significant overheating of the barrel, which can lead to thermal distortion, degradation of finishes, and faster wear. Depending on the application of the barrel, thermal stability may be more or less important.

AR Barrel Profiles

There are a lot of barrel profile options out there. We will focus on a handful of primary profiles.

Pencil

The pencil profile is the O.G. The M16 was designed with a pencil profile barrel for weight savings. The profile is by far the lightest of the standard profiles, because it has the most material removed.

Government

The government profile was invented to solve a perceived problem of warping and curling of the forward end of the barrel with long strings of heavy automatic fire. Whether or not those suspicions were real, the profile reflects the belief.

The government barrel adds a moderate amount of weight to the front end of the gun, which itself can be a benefit to managing muzzle rise. The added thickness on the front portion of the barrel does result in a moderate gain in stiffness, whether or not it is warranted.

This is our preferred profile for a combat carbine, because it contributes to a balanced feel of the gun.

M4

The M4 profile is very similar to the government profile. But the M4 profile only comes in a carbine length gas system, and it has a cutout turned ahead of the gas block journal for an under-barrel grenade launcher.

Heavy/HBAR/SPR

The heavy barrel (HBAR) profile, sometimes called SPR on rifle gas length barrels, is an ideal profile for a precision rifle that needs some degree of portability.

Less material is removed from the barrel blank between the gas port and chamber. This adds stiffness and thermal stability to the rear 2/3 of the barrel. The front end of the barrel is a little beefier than a government profile. But it doesn’t add the weight of a bull barrel.

The heavy profile is the ideal compromise between precision and portability.

Bull

Bull barrels have the least amount of material removed. They are practically an unturned barrel blank with some minor profiling for the gas block.

Bull barrels offer the greatest stiffness out of any profile.

Bull barrels are the heaviest out of any profile.

For both of these reasons, bull barrels are best for long range precision shooting. More specifically, they are for bench shooting. Carrying one of these beasts around gets old real fast.

Bull barrels offer great thermal stability. It takes a lot of rounds to heat one up. Given that most guns equipped with a bull barrel won’t be undergoing strings of heavy automatic fire, this means that the barrel will stay cooler for longer. After the first couple of rounds, the temperature will also be more consistent, which will lend to repeatability of your shots.

Fluting, Dimpling, and Other Weird Sh*t

Fluting refers to the process of cutting grooves into the barrel. These grooves may be aligned with the bore, may twist around the barrel, or may be cut in a pattern for aesthetics.

Dimpling refers to the process of cutting depressions into the surface of the barrel.

There is a belief that removing material from the barrel in a non-uniform way imparts magical properties on a barrel. The stated benefits are two-fold:

The barrel is stiffer AND lighter. This is partially true, but we consider it over-simplified, misleading, and mostly false. It is true that a fluted barrel that is turned to 0.750″ and then fluted to 0.625″ is stiffer than a plain 0.625″ barrel. However, it is also heavier than a plain 0.625″ barrel (because there is more material), and less stiff than a plain 0.750″ barrel (because there is less material). If you were to remove less material uniformly (e.g. turn a 0.700″ barrel instead of a 0.750″ barrel), you would also have a barrel that is stiffer than a 0.625″ plain barrel and a barrel that is lighter than a 0.750″ plain barrel. The benefits of fluting on stiffness is not as significant as it is sold to be. Unless you are shooting a benchrest rifle (and no AR is a benchrest rifle…) and shooting match ammo, you will not notice the effects of fluting over an unfluted barrel of the same weight.
The barrel cools faster than an unfluted barrel. This is true. The fluting (or dimples) increase the surface area of the barrel, which translates into greater exposure to the air. This allows more heat transfer to occur. However, most of the time, you will see fluting on barrels that don’t need it (i.e. you will see fluted barrels on precision ARs; not on full-auto ARs). As such, this benefit is not really applicable and is overstated.

If you are interested in a fuller picture, fluting and dimpling can do some other neat stuff too.

Because you are removing material, you are removing thermal mass. While this does mean the barrel will cool down quicker, it also means the barrel will heat up faster.
If not done perfectly, it will negatively affect barrel harmonics. Any miniscule imbalance in the distribution of the cuts will throw off the harmonics that will be hard to compensate for.
The cutting operation can introduce stress back into a barrel.
The process of dimpling or fluting adds significant cost to the barrel.
Having a barrel fluted or dimpled will usually void the warranty on the barrel.

We don’t use dimpled or fluted barrels. Ever. The positive effects are overstated (especially for an AR) and the downside is real. And with the added cost, this just doesn’t make any sense to us.

Gas Block Journal

The gas block journal is the part of the barrel around the gas port. The gas block is installed on this part of the barrel.

It is important that the gas block journal be very precisely milled. The tolerance between the gas block journal and gas block bore must be very tight to ensure gas efficiency. If the combined tolerance is too loose, gas pressure will escape between the gas block journal and gas block.

The gas block journal comes in 4 major sizes: 0.625″, 0.750″, 0.875″, and 0.936″.

The size of the gas block journal is influenced by the barrel profile. You cannot have a gas block journal narrower than the largest diameter on the barrel forward of the gas block journal (i.e. you will not have a 0.625″ gas block journal on a bull barrel).

The only real impact that the gas block journal size has is on the selection of the appropriate gas block. If your barrel has a 0.750″ gas block journal, you need a 0.750″ gas block.

Muzzle

When we talk about the muzzle, we mean the foremost point of the barrel. The muzzle end of a barrel can be everything from a clean cut end to a threaded portion for attachment of muzzle devices to an integral compensator.

Most AR barrels are threaded. Thread patterns can vary, based on the bore diameter and any conventions for a given caliber. The most common thread patterns for the AR are:

1/2″ x 28tpi (i.e. 1/2-28): This is the most common thread pattern for small frame barrels (i.e. AR-15).
5/8″ x 24tpi (i.e. 5/8-24): This is the most common thread pattern for large frame barrels (i.e. AR-10/LR308). It is also used frequently for AR-15 barrels with a bore over .25 caliber.
1/2″ x 36tpi (i.e. 1/2-36): This is most commonly found in 9mm barrels, though 6.8 SPC barrels may also have this thread pattern. Note that some 9mm barrels have 1/2-28 threading.