Did you launch a detent and spring across the garage during your build and now you can't find it? Aero Precision's Ooops Kit gets you the most commonly lost parts to finish your build. All parts in the Oops kit are compatible with AR15, M4E1 and M5 builds Includes: Takedown Pin Detent (4) Takedown Pin Spring (4) Bolt Catch Roll Pin (2) Trigger/Hammer Pin (2) Safety Selector Spring (2) Safety Selector Detent (2) Buffer Retainer (2) Buffer Retainer Spring (2) Disconnector Spring (2) 4-40 Set Screw (for threaded takedown pin detent hole) (2) Threaded Roll Pin