[J.B. Books]

"Friction being the highest of all the secondary force action on the wad/shot column as it moves down the barrel."

"My reasoning here is that the shot under acceleration of this force would try to spread out. Image trying to pile up shot on the top of your reloading bench the shot under just 1g of acceleration would rather spread out than stack."

http://mcb-homis.com/shotgunpressure...npressure.html