Well, CIP maximum is generally lower than SAAMI maximum so to get any decent performance CIP shells appear to need to be loaded closer to maximum peak under their standard.

The total work done by a shotshell is represented by the area under the pressure curve, so a lower peak pressure MUST be offset by some additional pressure down barrel someplace.

Charts 'proving' that this is 'not much' have been posted, but the fact remains that to get to 1200 fps for example, any shell loaded under any standard has to do the same work.

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