You would be looking at case head expansion and primers on the home front along with chronograph numbers on the manual; operated rifle just exactly like a bolt action rifle. The gas system is opening up when there is still pressure on the cartridge effectively turning it into a rocket hitting the bolt face ripping on rims and pounding the brass down into the ejector hole. The brass is hot and very elastic at that point. Hence the fact that you can fire ten rounds in the same AR chamber and get three or four different shoulder height readings. The earlier it opens, as in over gassed, the more violence to your case heads will be observed. The adjustable block or a heavier spring/bolt combination will mitigate some of these issues of observed over pressure indicators. Also be aware that a large firing pin tunnel in concert with a small firing pin will give false signs of pressure as this will induce cratering. Modern Remington BA rifles show this all the time with KNOWN lower pressure loadings.