I want to change out the stock upper on my Colt 6450 9mm AR for a flat top upper to mount optics on and install a rail system. Have any of you done so and if you have, what upper did you go with. It seems as though the Rock River 9mm A4 upper is the better choice with the deflector and dust cover already on. If anyone here did the swap, were there any issues that I should be concerned about? It seems pretty simple and straight forward as there is no gas tube to mess with. Thanks! Rob Wanna kill these ads? We can help!