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I know this thread is old but I was browsing slug threads. Snuffy is applying some good reasoning....but I think came to the wrong conclusion. Testing shows that slugs produce LESS pressure than the same weight of shot given identical components. The reason probably lies in the very thing Snuffy was discussing-shot charge fluidity. Shot is not truly fluid but in hydraulics/fluid dynamics pressure impinges equally in all directions. The "fluid" nature of a shot charge means that sidewall pressure against the wad petals/ barrel walls may approach breach pressures .Thus there is a LOT of friction as the charge starts up the barrel. A slug is more static. it may obturate some to fill the bore or swage down some in the forcing cone but it typically won't have nearly as much friction against the barrel walls on the trip up the barrel-at least that is the theory explaining why tests show reduced pressure when slugs replace shot as the payload.