Step 1: First, use an AR wrench to loosen the castle nut, making sure the teeth are facing to the rear of the weapon.

Step 2: Loosen the castle nut towards the rear and slide the castle nut and the lock plate towards the rear.
Note: Notice the takedown spring after loosening the lock plate, make sure the spring stays in place.

Step 3: Twist the buttstock up slightly and remove the takedown spring from the weapon. Place the spring in a safe location for later installation.

Step 4: Remove the takedown pin from the AR-15 and lift the upper receiver up from the rear.

Step 5: Locate the buffer retainer and the buffer retaining spring. Push the retainer down firmly. While holding the retainer firmly down in place, begin to unscrew the buffer tube from the lower receiver.
Note: Once you have removed the buffer tube, be sure to slowly release and remove the buffer retainer and buffer retaining spring.

Step 6: Remove the entire buffer tube from the lower receiver.

Step 7: Remove the existing castle nut, lock plate, action spring, buffer tube, and buffer assembly from the AR-15 buttstock.

Step 8: Insert ATI buffer tube into the buttstock.

Step 9: Assemble the action spring and buffer assembly by securing buffer assembly tightly inside the action spring.

Step 10: Locate the ATI castle nut and lock plate. Screw the castle nut onto the buffer tube with teeth facing the rear. Align lock plate with existing notch on buffer tube. Slide the lock plate towards the rear against the castle nut.

Step 11: Locate the takedown spring. Align and insert the spring back into place. Next, thread the buffer tube attached to the buttstock into place on the lower receiver.
Note: Before tightening the castle nut, be sure to align the lock plate to the lower receiver with takedown spring securely in place.

Step 12: Continue to secure the buffer tube into the lower receiver until it is locked in place and secure with the buffer retainer.
Note: A notch on the buffer tube will allow you to secure the buffer tube in place with the buffer retainer.

Step 13: Insert the action spring and buffer assembly into the buffer tube to the rear. Firmly push the assembly into buffer tube so that the head of the assembly is facing the front of the weapon.
Note: Holding the buffer retainer down with your finger or a screwdriver will help the assembly slide into place.

Step 14: Firmly push the action spring and buffer assembly into place so that the buffer retainer is holding the assembly in a spring-loaded position.
Note: Make sure the buffer retainer, buffer retaining spring, takedown spring, and lock plate are correctly aligned and in place before moving to step 15.

Step 15: Bring the upper receiver back down into place towards the rear. Insert the takedown pin back into the lower receiver of the AR-15, and lock in place.

Step 16: Use an AR wrench to tightly secure the castle nut and buffer tube in place.

Step 17: You have now successfully installed ATI’s AR-15 military aluminum buffer tube package. Enjoy!