Buffer parts for GEN 1 assemblies are NOT interchangeable with GEN 2 assemblies. Please ensure that you have the correct parts. Gen 1 parts can easily be differentiated from Gen 2 parts from the diameter of the recoil spring. If your spring is approximately the same size as the buffer tube and pushes on the back of the bolt without the spring going inside the bolt, then you have a Gen 2 assembly. If your spring goes inside the rear of the bolt, then you have a Gen 1 assembly.