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. 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.