The barrel is the most important part of a firearm. How the firearm will shoot depends on its parameters. The same firearm model can be produced with different barrels. And AR rifles, of course, are also made with different barrels. Let's put aside the design qualities of the barrel, and look at the technical parameters to understand what they affect and find out which barrels are best suited for certain tasks.
The first most noticeable parameter is the barrel length. The length of the barrel determines the initial velocity of the bullet ejected from the barrel when fired. The longer the barrel, the higher the muzzle velocity of the bullet. In .223 Rem, every centimeter of barrel adds about 5 m/s to muzzle velocity. Between 10.5 and 20 inch barrels, the difference in muzzle velocity can be up to 100 m/s.
The speed of a bullet affects its ballistic trajectory. The .223 Rem has an average muzzle velocity of 930 m/s, and since the .223 caliber bullet is fairly light, the bullet loses velocity fairly quickly. In general, if the firearm is used at close range, then the difference of 100 m / s does not play a special role. But at distances of 400-500 meters, this difference will already be significant. The ballistics of .223 and 5.56 NATO calibers allows you to more or less confidently shoot at distances up to 500-600 meters, longer distances for cartridges of these calibers are already too difficult.
The length of the barrel has little effect on the accuracy of fire, the lower accuracy of shorter barrels is noticeable only at long distances. Therefore, the length of the barrel should be selected based on the tasks. If the AR-15 rifle is planned to be used in Practical shooting, in recreational target shooting, then rifles with a short barrel of 14.5-16 inches will be more convenient, as they are lighter, more compact and maneuverable. The .223 caliber is rarely used in hunting or comparatively long range shooting, but if the AR-15 is specifically for hunting and target shooting, then it would be better to choose a rifle with a longer barrel. AR-10 models are usually designed for hunting, so they are made in other calibers, and come with 18-20 inch and even longer barrels.
Another, less significant, but also taking place moment: the shorter the barrel, the sharper and louder the sound of a shot from it, and the stronger the flash of flame. In sports, the sound of a shot and a flash of flame do not matter much, but in hunting it can already play some role. From this point of view, long barrels for hunting are also preferable to short ones, because. less scare game.
The next parameter is the pitch of the rifling of the barrel, the so-called "twist". This is the distance the bullet must travel through the rifling to make one full revolution. The steeper the rifling is made in the bore, the shorter the twist turns out, and the faster the bullet will spin when leaving the barrel. The twist of the bullet determines how well it will be stabilized in flight. Well-stabilized bullets hit the target accurately with the tip of the nose. Understabilized bullets in flight begin to “scour” and hit the target at random, sometimes even flying sideways. There is no need to even talk about any accuracy of the shot. There is also the so-called "restabilization" - too much twisted bullets when firing at long distances may also not hit the target accurately, they may also "roam" excessively during the flight. Therefore, for accurate shooting, the barrel and the bullet must be selected for each other. Longer and heavier bullets require a shorter (more “steep”, “faster”) twist, and vice versa, the lighter and shorter the bullet, the longer it needs a twist. Currently, AR-15 rifles in .223 are available with barrels that have 1:7, 1:8, and 1:9 twists.
In general, a 1:8 twist in .223 is well suited to 55-70 grain bullets, i.e. for most low-cost civilian .223 cartridges on the market, so for short-barreled carbines that are not intended to shoot long distances with great accuracy, a 1:8 twist is more suitable. If you plan to shoot with a long barreled carbine for high accuracy using expensive match cartridges loaded with a long bullet with high ballistic characteristics and a mass of 77 grains or more, then it is better to choose barrels with a twist of 1:7.
The fact is that there are two almost identical calibers: the civilian .223 Rem and the more powerful military 5.56 NATO. Outwardly, the cartridges of these calibers are no different, they are essentially interchangeable. But the chambers of these two calibers have differences. The 5.56 NATO caliber chambers are made more “freely” and with more gentle bevels of the rifling fields, which makes it possible to use even the most low-quality and cheap cartridges in them without problems and without delay. Of course, expensive commercial .223 Rem cartridges can also be fired from such a chamber, but the 5.56 NATO chamber will not give high accuracy of centering the bullet in the bore, and will not allow the cartridge to show high accuracy. For accurate shooting, the already “strict” .223 Rem chamber is needed. However, a rifle with such a chamber will also require more expensive and high-quality cartridges, if gross cheap cartridges are used instead, then delays in firing are possible.
Currently, 90% of the AR-15 rifles produced by manufacturers use barrels with a .223-5.56 multi-caliber chamber, which provide both acceptable shooting accuracy and sufficient reliability for working with bulk cartridges. Therefore, for hunting or for practical shooting, the multi-caliber chamber .223-5.56 is most suitable. And for high-precision shooting, the .223 Rem chambering in combination with the appropriate cartridges will be better.
Many manufacturers cover the bores of their firearms with chrome to give them wear resistance and durability. For AR-15 rifles, this is especially true, because when a .223 caliber cartridge is fired, the muzzle pressure of the caliber develops very high and the resource of a conventional non-chrome-plated barrel ends rather quickly, heat and erosion make the barrel unusable and it loses its accuracy. Therefore, AR-15 barrels are usually chrome-plated, while 5.56 NATO chambered barrels are also chrome-plated. The chrome-plated barrel "keeps accuracy" up to 10-15 thousand shots. The disadvantage of chrome-plated barrels is that their accuracy is slightly worse than that of non-chrome-plated barrels, since the chromium layer slightly impairs the geometric clarity of the barrel bore with its rifling. But this applies only to high-precision firearms. For conventional firearms, in which the resource of the barrel is important, from which intensive shooting is planned, and shooting for accuracy is not supposed, it will be better to have a chrome-plated barrel, its accuracy will remain acceptable for a long time. And match barrels with high accuracy (0.5 MOA and less) are not chrome-plated so that their accuracy is not lost. But the "high-precision resource" of such barrels is only 2-3 thousand shots. Of course, after this resource expires, the barrel will remain suitable for other types of shooting, or for hunting for a very long time.
In addition to chromium plating, there are other ways to increase the resource of the barrel, without a significant deterioration in its accuracy: carbonitriding, RF85.
Everything is simple here. Thick profile barrels are more accurate and retain accuracy longer when firing, as they heat up more slowly, but they are also heavier, firearms with such barrels are more difficult to hold. Barrels with a thinner profile are lighter, but their accuracy is lower than thick-walled ones, and they heat up faster. Therefore, thick match barrels are placed only on firearms designed for high-precision shooting.
AR-15 barrels are threaded (usually 1/2-28 TPI) for mounting muzzle attachments - flash suppressors, compensators, muzzle brakes and other devices. As a rule, rifles are supplied with a flash hider already installed. However, its function is to extinguish a flash of flame, nothing more. True, the flame arrester copes with this perfectly, even on short barrels. To install another device, you will have to unscrew the installed flame arrester. On American-made civilian AR-15s, the flash hider is usually installed very firmly, using fixation and even welding, so these points must be taken into account. If you are not sure, then it is better not to do it yourself.
What is the best stem?
The AR-15 barrel is selected based on the tasks that will be assigned to the rifle. If the rifle will be intensively used in shooting ranges, in practical shooting, hunting, then it is better to take a rifle with a chrome-plated short, thin barrel - 14-16 inches, with a 1:8 twist, and a multi-caliber chamber .223-5.56. And if the purpose of the rifle is high-precision shooting, then a longer match (thickened) non-chrome barrel with a 1: 7 twist, and a strict .223 Rem chamber would be better.
But remember that it is impossible to give unambiguous recommendations for all barrels, since a great many barrels are produced for the AR-15, and a wide variety of options are possible, so what is written in the article is not a dogma. This is just some averaging of the facts. The article gives concepts about the main parameters of the barrel and their influence on the barrel characteristics. In the future, in order to obtain better results in shooting, it is necessary to increase your knowledge of external and internal ballistics.