You need this kit if you're modding an accutrooper (alpha trooper). From my understanding the stock priming bar is very weak, so this is a necessity even if just upgrading the spring. I combined this with the worker short dart kit, using this spring instead (for durability) and it hits a lot harder now. I really enjoy the alpha trooper now. If you have an accutrooper/alpha trooper and are modding it just get this and don't worry about the priming bar!