The Hera Arms SFU Buffer Tube Folding Adapter was developed for the Hera Arms Triarii system, but it will work with any system using an AR-15 receiver extension tube that does not use an internal buffer and spring. It's great for bolt action folding skeleton stocks designed to accept AR-style receiver extensions and for pistol brace applications (other than AR-15s). This adapter WILL NOT work for standard AR-15s that require a buffer and spring to cycle within the extension tube. Note: This adapter is not for standard AR-15s that require the use of a recoil buffer and recoil spring.