The KNS Precision AR-15 Perma Firing Pin Retainer was inspired by the design of the retaining pin found in the original AR-15 prototype developed by Eugene Stoner. The pin retainer ensures that the firing pin remains secured into place and works with any standard AR-15 bolt-carrier group. The pin retainer is made from heat-treated steel, which provides flexibility and strength. The pin cannot be used on .308 bolt-carrier groups.