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/H/H2 Stealth Recoil Spring Kit: Manufacturer: Armaspec Color: Stainless/Black Gun Make: AR Platform Gun Model: AR-15 Length: 7.8 in Condition: New Country of Origin: USA Package Contents: Armaspec AR15 Gen4 C/H/H2 Stealth Recoil Spring Kit