The Stealth Recoil Spring 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. NOTE: The new GEN4 version does not have the o-ring on the weight. To eliminate any potential failure points, we changed the screw to a roll pin across the plastic end piece, and the cap is machined on the rodm replaced the snap ring with a washer. Compatible with AR-15 with carbine-length buffer tube. AR10 version is available, please find under AR10 parts. Most bolt carriers have a rear opening of over .60″, there are a couple of bolt carriers in the market place that have a rear opening of under .60″ and will not function with the SRS. Specifications for Armaspec AR15 Gen4 C Stealth Recoil Spring: Manufacturer: Armaspec Color: Stainless/Black Fabric/Material: Steel Gun Make: AR Platform Gun Model: AR-15 Length: 7.8 in Weight: 6.2 oz Country of Origin: USA Features of Armaspec AR15 Gen4 C Stealth Recoil Spring Drop-in replacement for standard buffer and spring Reduce felt recoil Keep the buffer spring from rubbing against the inner wall of the buffer tube Buffer mass is designed to stay under constant tension against the bolt carrier Rubber O-ring on the head of the buffer mass prevents metal on metal contact made in specialized factories Package Contents: Armaspec AR15 Gen4 C Stealth Recoil Spring This product is compatible with the following gun models: AR-15