This Smith & Wesson Striker Block Plunger is made from 17-4 and heat treated for increased durability and longevity. It is a replacement for your factory one. This fits M&P models M&P 9, M&P 9 compact, M&P Shield 9, M&P 40, M&P 40 compact, M&P Shield 40, M&P 45 and M&P 45 compact.