Kaw Valley Precision Extra Power 17-4 SS Pistol Caliber Carbine Recoil Spring, Stainless Steel, Medium, KVP-PCC-XP-SPRING

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.