If you buy a PSA lower it will mate to the upper. The PSA upper will not fit up to an aero lower. Are you using a carbine stock or an A2 rifle stock. That answer determines which buffer and spring to use. You still will probably have a to short gas tube and a rifle that is only so so accurate. I'm not knocking PSA just stating facts. I have 2 of them and they work and shoot now. I would return the PSA upper and build an upper with Aero parts. Cheaper in the long run.