Per 2-p and JW. Part of the primer "fire" is absorbed in warming up the powder and hull base. There is a point for each load where too much "fire" is absorbed and not enough left over to raise the pressure and temperature in the hull "combustion chamber" to the point where the powder will reach normal pressure in the correct time. If the wad moves too much before the powder reaches needed pressure, the load will not reach pressure and you get a blooper; the pressure can't catch up to the expanding combustion chamber volume.
