Not needed for regular maintenance, but nice to have if you want to swap out parts (e.g. flash suppressor, gas cylinder, gas cylinder lock, barrel band, or op-rod guide). If you want to get one, stay away from the cheap knock offs that Brownells sells. They will bend and/or break when attempting to apply the amount of pressure needed to loosen and tighten the castle nut. Sherluk's sells the USGI kind, and the commerical copies that Springfield sells are good quality.