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Because the modern wad is a relatively soft plastic, you can see the imprint of the shot in an expended wad. If you pick up a couple after you shoot, you will note the permanent impressions of individual pellets and this testifies to the force of the explosive burn that the wad must accommodate. This is caused by the vertical unevenness of the movement through the barrel. The rear shot pellets move prior to those at the head of the shell and this force shows in the spent shotcup. (For someone who has never quite understood how the inside of a wheel spins at a faster speed but at the same rpm as the outside of the wheel, this is all very mystical!) Perhaps you would expect this since within a millisecond, the pressure builds to around 10,000 psi and the load achieves terminal velocity within a few inches of travel.

The writers of a former edition of this book, Kurt Fackler and M.L. McPherson, likened the effects of the powder burn to a shock wave, causing the bottom layers of pellets to “squash themselves against the upper layers.” This shock, they said, was the equivalent of wrapping a towel around a hammer. The instant of most extreme shock and acceleration, which moves the pellets from zero fps to maybe 1,200 fps, is cushioned.