As described by Emary in an article apparently no longer available on the web (formerly at http://personal.stevens.edu/~gliberat/carcano/emary.html ), the mechanism behind the pressure anomalies is that the bullet has to be driven into the lands with sufficient force to engrave without hesitation. Reduced charges that fail this requirement allow the bullet to hesitate upon reaching the end of the long Carcano throat, producing a hazardous pressure spike.