crimps
From the Accurate reloaders Guide - Crimping: This is certainly one of the most important aspects of the shotshell reloading process. The influence of crimp on the ballistics is often ignored, and assumed to be of lesser importance than primers and wad make/design. The fact is that the effect of Crimp-strength can totally overshadow the influence of the other components and parameters. This is controlled by the following: Crimp depth, condition of case, wad length and condition.
Some reloaders want to extend case life and they tend to crimp as shallow as possible. However this pratice can be problematic if a "soft" combination is used it can lead to underperformance or in extreme cases "bloopers".
They go on to say the difference between a shotshell and a rifle is the rifle cartridge gets its resistance and Maximun Peak Pressure from the bullet being engraved from the rifleing where as the shotshell gets its from the crimp. There for, the shotshell Maximun Peak Pressure is reached long before the base of the shot/wad assembly has left the case. From what I've read in other articles, reloading data is with a .060 deep crimp. A .020 depth would have about 2000psi less pressure.
Just thought I'd give you guys something to think about. Maybe that's why I had trouble with nitro powders in Brass hulls and glueing in the over shot wad - too much volume for the powder and no resistance for load. Paul
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