The TangoDown AR-15 Castle Nut Wrench has a 3/8" drive hole on either side of the wrench, laser marked for proper use. Use a breaker bar on the left for Nut removal, or a Torque wrench for proper tightening on the right. Either direction forces the wrench down onto solid contact with the Nut (unlike wrench lifting away from the nut like on some other designs) for no-mar, positive installation or removal.The Castle Nut teeth are dimensioned for positive, non-slip engagement that also prevents damage. Precision machined from 4140 steel and heat treated to 40 Rockwell for long life, finished in a durable black oxide finish. Try one, and you'll agree this is one of the best tools you can own for high end AR builds.