A firearm having a lower receiver is provided. The receiver is adapted to couple with a plurality of upper receivers having different calibers. The lower receiver comprises a receiver extension having an opening therein. A spring is disposed within the opening. A buffer member is provided having a first portion having an end. The buffer member is coupled to the spring within the opening, wherein the buffer member is movable between a first position where the end is within the opening and a second position wherein the end external to the opening.
