The krag rifles have a long throat meaning free bore that has no rifling. This space causes the two pressure spikes in a pressure reading. So a rifles ability to hold a given pressure for a given amount of time. Time being the key measure here. The bullet jumps the lead or throat of the chamber to the rifling where it now is graving into the rifling causing it to slow momentarily, thus our second pressure spike. With normal loads to loads in the high to maximum range you probably won't notice much if anything and the spikes will be close to the same pressure. Now back to time, if you try a load that is at or below standard loads the time at pressure now jumps even doubles and where is it at, right in front of the lead or free bore and the burning powder behind it now jumps to full pressure and it has a pressure wave in its way!!!
The rifle then goes boom.
SB