cpa the pressure at the bottom of a 1000' column of water is ten times higher than the pressure at the bottom of a 100' column of water. The reason is that there is 900' more of water stacked on on the bottom (and water is practically incompressible).
Stand a 12 gauge empty cartridge and a 28 gauge empty cartridge on their base on a table. Fill each with 1oz of #9s. The 28 gauge shot column will be taller/longer than the 12 gauge. Suppose the 28ga column is twice as tall/long (I don't know what it would be).
When you accelerate that shot from stationary to 1200 second in a fraction of a second that shot is under huge acceleration, or Gs, as they say in aerobatics and at NASA. A "G" is one earth gravitational field. The water columns in my first example were under one "G". The shot is experiencing dozens or hundreds of Gs, again I don't know the number. But the shot at the bottom of the shot column is pushing the next layer of shot which is pushing the next layer shot which is pushing the next layer of shot. Just as in the ocean the "pressure" of the shot at the bottom of the shot column is experiencing a much higher pressure the the shot at the top of the column.
Edit: And the taller/longer the shot column the more the pressure on the bottom layer of shot.
