This receiver extension is designed to work with the VLTOR A5 system. It is longer than a traditional M4/AR15 carbine receiver extension, and is not compatible with standard (3.25″ OAL) carbine buffers or carbine springs. An A5/rifle length spring and VLTOR A5 buffer must be used with this receiver extension.
The outside diameter of this tube is mil-spec, and it will accept all mil-spec dimensioned M4/AR-15 type collapsible stocks. Some smaller, closed-back stocks, like the VLTOR Imod, won’t fully index into position #1 on this 7-position tube, but will on the 6-position RE-10/A5SR.