Smith & Wesson M&P15-22 Firing Pin Retaining Pin, 3/32"x5/8". Produced from steel, this 3/32" x 5/8" firing pin retaining pin is manufactured for the Smith & Wesson M&P15-22 blowback-operated rifles chambered in .22 LR. Inserted into the side of the bolt assembly, this pin is responsible for retaining the firing pin within the bolt. This part is perfect as a spare on the workbench or as an immediate replacement for a lost or damaged piece.