Pistol caliber carbines such as 9mm, .40 S&W, .45 ACP, and 10mm are a blast to shoot but can be tough on AR-15's internal components. Without a properly tuned blowback system, you run the risk of being overly destructive on internal lower parts. Running a blowback AR15 with a suppressor can further increase the risk for damage. In addition to a properly weighted buffer, the KVP Extra Power recoil spring is a must! The KVP XP 17-4 SS spring offers increase pre-load over standard springs. The additional preload helps increase the Blowback systems dwell time. FeaturesOptimized for 9MM, 10MM, .40 S&W, .45 ACP, and .308 blowback and AR platformsIncreases AR-15's dwell time in blowback setupsIncreases Pre-loadQuiets action (better for suppression)Reduces felt recoilDesigned only for Carbine Length Stock and pistol kits.