Ralph, rather than pressure I believe that it is gas volume that drives bullet speeds. A pressure peak of for example 2 nano seconds might produce less gas than one with a pressure peak of 5 nano seconds that has a lower pressure. That is why 5.6 gr Ramshot Sillhoutte produces 1050 fps from a 4" bbl 38 special and a 158 gr sec. The pressure is listed as 16, 800 psi.

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Front sight and squeeze