ON a tangent, I read around here somewhere that having gaps between the powder and the ball could be very dangerous, but I can't imagine why--wouldn't such a condition merely reduce the amount of pressure in the chamber when the gum was fired?
I do not understand the exact physics, but here is my educated guess:
First of all, I would think that the pressure at ignition starts high and then reduces as the bullet travels down the barrel. There is probably some ramp-up to peak pressure and then a reduction as the bullet moves.
With a bullet fully seated, this maximum pressure is distributed over a smaller area of chamber. 20,000 pounds per square inch is only a little bit of force over a small area, but can become a large amount of force over a large area. So if there is a gap under the bullet, you have effectively distributed the same amount of pressure over a larger area of the firearm, resulting in the firearm bearing more force.
Another possibility is that the air under the bullet is itself compressible. Perhaps having a larger volume of air results in higher pressures somehow.