The AR15 rifle, which is very similar to the M16 rifle or the M4 rifle, consists of two major parts that are commonly referred to as the Upper and the Lower receiver. In this article we will describe the parts of the Upper and how it is used in the AR15 with semi automatic fire.
The AR15 complete Upper consists of the upper receiver, the Barrel, sights, firing pin, charging handle, forend grip, the gas system – including the bolt/carrier, gas key, and gas tube.
- Upper receiver – this part is what the lower receiver connects to, it holds the bolt and carrier. The rear sight is usually either part of this or attached to this. In the case of the A1 and A2 the carry handle rear sight is part of the upper receiver. With the A3 upper it has a picatinny rail that allows for sights to be attached to it, such as holographic sights or scopes.
- The Barrel – The AR15 barrel is different than most rifle barrels, except those with similar gas systems, the barrel is usually purchased with the barrel extension attached, it comes with a gas port on the barrel that allows for the gas to be expelled and used to eject the casing. Barrels come in different contours or types, such as heavy, light, M4 style, etc. The end of the barrel that opposes the barrel extension can either come threaded for a flash hider, compensator, or other attachments, or can come with a target crown that is not threaded and has been cut in a smooth bevel to aid in the equal dissipation of gas as the bullet leaves the barrel, for accuracy.
- Sights – The AR15 can have a large variety of sights, open or iron sights are common, especially with A1 or A2 style. With an A3 style the sights can be electronic, optic, lasers, or a combination of different sights.
- Firing pin – The firing pin is a part of the bolt and carrier which resides inside the upper receiver.
- Charging Handle – There are many styles of charging handle, they are used to pull back the bolt and carrier to allow for the first round of a magazine to be loaded into the chamber. Once that is done the charging handle is not used during firing.
- Forend Grip – The AR15 can have many different styles of forend grip, this is where one of the hands can rest to aid in stabilizing the barrel. They can be plain, smooth, tube, quad picatinny rail for attachments, free float, etc.
- The Gas system – The gas system of the AR15 is what is used to eject and load the next round during semi automatic fire. As the bullet goes down the barrel it will pass the gas port in the barrel, which is a hole on the top of the barrel, the gas that is following the bullet will be pushed into the gas port and back down the gas tube, going into the gas key and into the carrier. The gas then pushes on the gas rings that are on the bolt forcing the carrier to twist the bolt, unlocking it, and then pushing the bolt and carrier back into the buffer tube. In the process it will eject the spent casing, the new casing will then pop up and as the carrier comes forward it will push the new live round into the chamber and the bolt will then lock into place ready to fire. The hammer will also be pushed back as the carrier goes back into the buffer tube, cocking the hammer so that it is ready to fire as soon as the carrier is back in place and the trigger is pulled again by the shooter.
Here is a great book to aid in assembling an AR15 Upper: The AR-15 Complete Assembly Guide