I am most skeptical that the bullet could possibly create more volume by its movement than the powder can fill with gas. I think that's the flaw in your theory.
Any convexity in the curve after the absolute peak is due to the "tailoff" of residual propellant, but that is a minor and short-lived event, as evidenced by the faster (and concave shape) shortly after the peak.
The bottom line is that pressure builds as long as there is deflagration. By definition then, when the pressure ceases to build, burning has (essentially) completed.
