Buffer Spring

The Buffer Spring pushes the buffer forward and absorbs the recoil of the bolt carrier group as it is pushed back from the gas of the bullet.  It then will push the buffer forward that pushes the bolt and carrier group forward, the bolt will catch another round from the magazine and load it into the chamber.  Here is a picture of a Buffer Spring:

Buffer_SpringYou can purchase a Buffer Spring from Brownells here, from Amazon here, or from MidwayUSA here.

In this picture you can see the installation into the AR15 lower.

Buffer installation ar15