The AP 308 Carbine Receiver Extension is slightly too long, internal depth.
This upper and lower had never been fired, only hand cycled. I notice marks on the lower where the BCG just barely touched it a couple times. Layed everything out and started measuring. The internal depth on the receiver extension is 7.100" deep. The buffer is correct, at 2.500" long. The spring is correct. That internal depth is just enough that the BCG just misses contact with the lower, when you push the BCG into the lower by hand. Any type of buffer bumper compression is gonna cause the BCG to hit the lower. Recoil will definitely do it.
Checked the amount of distance from the bolt face to the bolt catch, with the BCG all the way back. There's an excessive amount of distance that the face of the bolt goes back, beyond the back of the bolt catch, more than 1/4" (over 0.0250"). This is what breaks bolt catches.
I'm not going to start replacing parts yet. I'm going to shoot it. However, I'm going to make sure that I don't rip up bunch of nice parts, too. I knew I needed to take up some BCG travel, to prevent the BCG from hitting the lower receiver. 2 quarters down the receiver extension will do that. In playing with the overtravel beyond the bolt catch, I can get another 2 quarters in there, but not another 3. We'll see of it locks back on $1.00. I might need to remove one, if it won't lock back. We'll know after the range session.
Those are the issues I've seen with this combination of parts. All major parts are Aero Precision (upper, lower, receiver extension, buffer, buffer spring, charging handle, lower parts kit, upper small parts).