SOPMOD M4 is supported due to its metal frame and its shoot and recoil engine gimmick. Because of these realistic truthful features, it also runs into some problems. The hole for trigger lock pin on the metal frame often becomes loose due to multiple stripping and the stress from the recoil shocks, and many end up losing the pin. By using screw fastening method, this lock pin will stay fit no matter how lose the hole becomes. Low head type pin to minimize the stick out to reduce interference, and the hex hole head design adds taste.