The kac barrel extension action wrench is used to securely hold an sr-15 or sr-25 upper receiver in a bench vise so that work may be performed on the muzzle device and/or rail system.It is inserted through the rear of the upper receiver to engage the barrel extension.*note- this tool is compatible with sr-15 e3 and sr-25 barrel extensions only.It can be damaged by using it in a standard square lug bolt ar-15 barrel extension.