It has been extablished various authorities (Homer Powley among them) that the peak of the pressure curve is where all (or almost all) of the powder is burned. As I recall, this was established partly by cutting back barrels and seeing how much unburned powder was left at certain lengths.
Some people are under the impression that the peak is caused by the bullet fully engraving in the rifling, but that happens almost immediately in front of the chamber.
