Installing the fire control group of an AR-15 rifle is actually one of easier steps. Though there are many parts that have to be assembled correctly, the process of doing putting together the hammer and disconnector and installing it and the hammer into the receiver is not difficult at all. Watch the short video to the right and take note of the orientation of the trigger spring and hammer spring and you’ll do just fine.
First, install the trigger spring onto the trigger. Take note of the orientation of the spring shown in the video to the right. Insert the disconnector spring into the trigger and make sure that the larger portion of this tapered spring is placed down into the trigger.
Install the hammer spring onto the hammer.
Position the trigger spring so that the squared off portion is on the underside of the sear in front of the trigger. Drop the trigger into position in the receiver with the legs of the trigger spring facing forward. Place the disconnector onto the trigger. You’ll need to apply pressure to the trigger assembly in order to line up the holes of the disconnector and trigger with the corresponding hole in the receiver. With the assembly lined up, insert the trigger pin. You may need to gently tap the pin into place using a brass hammer in order to seat it flush with the receiver.
Test the trigger for proper movement. It should move smoothly and without binding.
Place the hammer into the receiver with the legs of the hammer spring on top of the trigger pin. You will need to press the hammer forward and down to line it up with the holes for hammer pin. Insert the hammer pin into the receiver. The pin should meet some resistance and may stop halfway through and need to be tapped into place with a brass hammer.
Pull the hammer back and function check the trigger to ensure that the hammer is held back until the trigger is pulled. When function checking the fire control group, hold your thumb or palm over the hammer to prevent it from flying forward. Do not allow the trigger to strike the receiver, as it could cause damage. Test the disconnector by holding the trigger back while you cock the hammer: the disconnector should engage the hammer and allow it to drop onto the sear when you release the pressure on the trigger.