VLTOR Weapon Systems Receiver Extension Tube, Fits M4/AR-15 Stocks, 7 Position, Black RE-A5 The RE-A5 Receiver Extension is a 7-position receiver extension tube designed to work with the VLTOR A5 buffer system. This receiver extension is slightly longer than the traditional M4 receiver extension and is designed to work with Mil-Spec dimensioned M4/AR-15 type collapsible stocks. This receiver extension tube can also can be utilized as a replacement receiver extension for the AR-10 rifle.