I think 'total impulse' might be a better term Miller.

That is what is represented by the area under the pressure curve. "The work done".

If the pressure never reaches the intended initial value, the area will likely total lower.

Crimp condition and crimp depth both affect initial pressure rise. The shell closure resists the initial payload movement thus allowing the pressure to build up to the design optimum for the powder charge.

A new shell is best, and it's downhill after each firing.

Is it a problem? Not a huge one since shotgun powders are on the fast end of the range of smokeless powders, especially those used for target loads.

Where crimp problems might get serious is when using slower powders in cold weather. It's best to use only new or once-fired hulls for field loads.



"The price of good shotgunnery is constant practice" - Fred Kimble