There is an approximate correlation between MV and peak chamber pressure, but that is affected somewhat by the propellant used, and is not very reliable. Extreme overpressure will cause expansion of the cartridge case head diameter, i.e., the area just ahead of the rim or extraction groove. But when you see an increase in diameter, you've already gone too far. I won't go into detail, but I once had a project to develop some 5.56mm loads, and I had no other way to judge excessive chamber pressure than to measure the head diameters with a digital caliper before and after firing. When it increased more than about 0.002", I stopped. Judging pressure from primer appearance is very imprecise and unreliable, except if you see primer flattening and firing pin cratering, you know it's a hot load. If the primer blows out of the pocket, you know it's really hot.