AR-15/M4 Collapsible Stock Spanner Wrench can be used onto either AR15/M4 rifle or a shotgun stock adapter. Used to loosen or tighten the Castle Nut (Lock Nut) that secures the collapsible stock tube to the receiver or adapter. Spanner wrench Made of stamped steel. The wrench acts as a barrel bushing wrench on 1911 government model pistols also. Limited Lifetime Warranty.