Denton, alternate theory from Kikkonen, slightly embellished, no idea if it's right.
Slow powders with a heavy deterrent coating are hard to ignite. Given a large air space in the cartridge and the powder not necessarily near the flash hole the energy of the primer would to a greater extent dissipate rather than couple into the powder. Further any gas from burning would have more volume in which to expand, slowing the rate of pressure (heat) increase. The powder does not ignite cleanly and "smolders" which produces explosive products of nitrocellulose decomposition. Heat from compression, burning through the deterrent coating, or something else causes chamber temperature to rise and ignite the explosive gas.
In favor of this theory Kikkonen noted that there was something less than a 100 millisecond delay between the primer firing and the explosion in the .243 example. He also wrote that people experiencing what was likely S.E.E noted a slight hang fire.
Again, I don't know.
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Also in working with very light loads (with very fast powder) I noted a point at which powder residue reduced markedly. This correlated with a small jump in velocity, below and beyond which velocity increased more or less in proportion to powder charge. Not that this necessarily means anything, particularly since it wasn't under controlled conditions.
Last edited by nighthawk; 04/29/08.
