A pressure curve that shows higher pressure at the point of the standing breech than at the point in the chamber where the powder charge is situated in the shell should be viewed with extreme skepticism. After all, it is the rapid burning of the powder, which sits between the base wad and the under shot wad, which generates the pressure. Peak pressure will obviously be generated very near that point. We don't see examples of early pressure test barrels with lead crushers at each and every point along the full length. So it should be obvious that there was considerable interpolation and conjecture involved in the early pressure curves. And we would also have to assume that even the modern computer generated version relies on some unknown technology that would provide continuous pressure data at every point as the charge passes down a 30" barrel. We would similarly need to have piezoelectric transducers installed along every point of the barrel to know precisely where the shot charge was at a given time in the pressure curve. Multiple strain gauges could be installed along the full length of a barrel, but they are not as accurate as piezoelectric transducers which require a hole in the chamber wall. As it stands, we rely on the ideal gas law to know approximately where the shot charge is at a given point in time. But the pressure is variable, dependent upon the burning rate of the powder in a confined space of continuously variable volume.
