I think we're talking about force applied to the shot charge which is subject to Newton's law - not water in a column. Per square inch the pressure in the 20' column would obviously be less because there is less mass above it - 240 cubic inches versus 1200 cu. in. In the shot columns, both have the same mass, and volume - they simply have different height. Likewise the 28 gauge shot column in your example has exactly the same mass as the 12 gauge. It just has smaller area, but the cubic inches of shot and mass are the same.

Last edited by cpa; 04/14/16 08:52 PM.