I have roughly 1,000 rounds through my AR after going to a few carbine classes. Most of the carbon buildup is at the bottom corners of the upper (near the barrel extension) and the edge of the BCG behind the bolt. All the critical areas like the cam pin and the “rails” of the BCG and upper have very little carbon since they continuously contact and move against these parts.
If these areas stay fairly clean, where will the carbon buildup cause issues?
