[Paul Harm]

From a Accurate Powder Co. reloading handbook :
The fundamental difference between a shotshell and a typical center fire rifle cartridge is, that the efficiently of the shotshell is 100% dependent on the round itself. By this we mean that all the " resistive forces " must be generated within the confines of the round itself. No assistance is provided by the gun.
The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the the case. [ In the case of a CF rifle cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of leade/free-bore dimensions on the combustion process. [ ie. bullet/bore interface fit, bullet hardness, bearing surface, etc].
The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle cartridge is totally absent in a shotgun.
This means that the efficiency re ignition and the subsequent increase in pressure, is totally controlled by the integral configuration and assembly of the round itself. These constitute the main internal mass [ shot mass ], the internal volume [ wad design ], the dynamic collapse [ primary expansion ] of the internal volume [ collapsible section of the wad], plus the displacing of the internal assembly and the unfolding of the fold/crimp [ secondary/final expansion].
The way this COMBINATION interacts, will determine the efficiency Pressure impulse [ profile and time-base ] and the Peak-pressure vs Velocity [ P/V ]. The resistive force, presented by friction in a shotgun is negligible.
Crimping is certainly one of the most important aspects of the shotgun shell 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 the case, wad [ stiffness ], and wad tension.
With that said, from a article I once read, and don't have now, factory ammo has a crimp depth of around .050. When you read reloading data [ pressure] it's with a .050 crimp depth. Any reload with less depth will have less pressure, and deeper will have more pressure. If one likes less crimp in the hopes of increasing shell life, and loads at low pressures, bloopers could result. Or, if someone wanted to have a bit less pressure from given data, he could go to a .030 or .040 crimp depth. How much - I don't remember the numbers but, and this is a but, if only .020 was used the load would be reduced by 1000psi - but don't quote me. I do remember it quite a bit more than I would have thought. I'm sure the same applies to roll crimps. And that brings to mind something else - maybe that's why I could never get good reloads with brass cases unless I used black powder, and maybe that's why Charlie got good loads with nitro by gluing in the over shot card - it gave the resistance necessary for good combustion. Just some food for thought.