I had to put a lighter spring in my complete sized M16 A2 clone nevertheless it ran wonderful. Tried using again Using the 11.five" Comando Colt upper. No dice. Ran 500 rounds of wolf through it today. Didnt damage the spring in any respect. It utilizes a “forced reset” system, https://archerwszzz.bloggazza.com/35990007/frt-15-fundamentals-explained