Most AR-15 barrels come partially assembled. Let's take a look at the parts that should be included.
The barrel assembly is made up of the barrel, the barrel sleeve, the barrel nut, the gas block and front sight base and taper pins. Because installation of these parts requires extreme precision or pressure, most manufacturers perform this work for you. Let's start with a partially assembled barrel. Before we can attach it to the upper receiver, we need to install the delta ring, the weld spring, and the retaining ring. Slide the delta ring onto the barrel from the rear over the barrel nut.
Followed by the weld spring.
The retaining ring is last.
Slide it onto the barrel nut and using a pair of snap ring pliers. Open the ring, engage it in the groove on the barrel nut, and release the pliers.
That's all there is to it.