The barrel is the heart and soul of any firearm, the anatomy of the barrel may sound foreign to new builders. In this video we will go over the basic parts of an AR15 barrel and their function. This terminology also applies to AR308/M5 Barrels.
The threads on the muzzle of you AR15 barrel are used to attach any sort of muzzle device such as a flash hider, muzzle brake, compensator, or suppressor to your barrel. There are several different types of muzzle threads (distinguished by "thread pitch") that vary depending on the bore diameter of your barrel. On most standard barrels, a rule of thumb is that 5.56/.223 barrels have a 1/2x28 thread pitch, whereas 30 caliber barrels have a 5/8x24 thread pitch. If you are unsure what pitch muzzle threads your barrel has, consult the manufacturer of your barrel.
The gas port is a small hole drilled into the 12 o'clock position of your barrel. The gas port has a very important job in cycling your AR pattern firearm, as it directs the necessary gas from the barrel during firing into your gas block and gas tube.
The area of the barrel where your gas port sits is often referred to as the "gas block journal". Depending on the profile of your barrel, you may have a different diameter gas block journal. This size will correspond to what size gas block you need.
You will notice a "shoulder" just before the gas block (towards the rear of the barrel). This shoulder is to help index your gas block on your barrel by serving as a reference point. Your gas block will sit approximately .025” off of the shoulder (this extra space is to accommodate a handguard end cap if utilizing a traditional two-piece handguard).
The last thing your gas port will dictate, is what length gas system your AR15 barrel is setup for. The 4 most common gas system lengths are in order from shortest to longest: pistol length, carbine length, mid-length, and rifle length. You will have to purchase the appropriate length gas tube to correspond to this length when building your upper receiver. If you are unsure what length your gas port is drilled at consult the manufacturer of your barrel.
The barrel extension is a part of the barrel that is threaded on during the manufacturing process at the rearward end of your barrel. This has several functions. The first is, it gives you an indexing pin at the 12 o’clock position that engages with the upper receiver, this ensures that your barrel is properly timed when being installed on your upper receiver.
The second major function that this has is provides a place for your bolt lugs to engage with when the firearm is in battery. You will often hear the inside of this component referred to as a "star chamber".
Starting at the rearmost part of your barrel, just past the barrel extension, is your chamber. This is where the cartridge will sit in your barrel. The chamber in conjunction with your bore diameter is what determines that round that your AR15 will shoot.
Just past your chamber is the bore itself, this is where the projectile travels when the gun is fired. The bore has a series of groves in it that “twist” down the barrel, this is referred to as "rifling". The rifling is what imparts a twist on the projectile to provide in-flight stability. Over a very long period of time your rifling can wear down, the speed in which this happens is dependent on a lot of factors including the type of ammo you shoot and care/maintenance.