The Kentri 3.5 is a direct, drop-in replacement for your traditional carbine buffer, buffer spring, and receiver extension, extending just 3.5 inches from the rear of your receiver. No modifications required. This specific model is only compatible with carriers that use a non-integrated bolt (traditional blowback pistol caliber carbines will have different internals, coming soon). Compatible calibers include 5.56x45mm, .300BLK, and 7.62x39mm. CMMG Banshee 9mm radial delayed blowback BCGs are also compatible with this model. An M4/M16 bolt carrier is recommended for reliable function (lightweight carriers may lead to malfunctions). The Kentri 3.5 (KTR35N) includes a tube, dual-spring recoil assembly, and end cap. You will need your own bolt carrier group, castle nut, and receiver end plate to complete installation. Specifications for Pantheon Arms Kentri 3.5N AR-15 Short Buffer System: Manufacturer: Pantheon Arms Color: Black Finish: Type 3 Hard Coat Black Anodize, Black Oxide Fabric/Material: 7075T6 Aluminum/Stainless Steel/Alloy Steel Gun Make: AR Platform Gun Model: AR-15 Gas System Length: Carbine Length Diameter: 1.3 in Length: 8.05 in Condition: New Dimensions: 1.80 X 1.90 X 8.05 in Cartridge: 5.56x45mm NATO, .300 AAC Blackout, 7.62x39mm Height: 1.8 in Features of Pantheon Arms Kentri 3.5n AR-15 Short Buffer System Direct drop-in replacement for your carbine buffer Buffer spring and receiver extension Only compatible with carriers with a non-intergrated bolt Will not work with Blowback pistol caliber carbines Not compatible with LW carrier Package Contents: Pantheon Arms Kentri 3.5n AR-15 Short Buffer System