The Armaspec Stealth Recoil Spring SRS is a self-contained multi-stage drop-in replacement for your standard buffer and spring. The SRS is designed to keep the buffer spring from rubbing against the inner wall of the buffer tube providing smooth, quiet operation and the second stage helps reduce the felt recoil significantly. FeaturesDrop-in replacement for standard buffer and springReduce felt recoilKeep the buffer spring from rubbing against the inner wall of the buffer tubeBuffer mass is designed to stay under constant tension against the bolt carrier Notes:Most bolt carriers have a rear opening of over .60a ^2, there are a couple of bolt carriers in the market place that have a rear opening of under .60a ^2 and will not function with the SRS.9mm Bolt Carriers ONLY: For proper function of the SRS-9 in your 9mm rifle or pistol, the extra weight on the back of the bolt carrier needs to be removed. It is held in with a spring pin.Big Bore version is designed for large AR-15 ca...