Greg,
This is mysterious and I do not completely understand the causes of detonation is a shotgun or pistol. I am not certain anyone does...
Detonation is an explosion rather than the deflagration (buring) of power. High explosives detonate via a supersonic shock wave. For low explosives such as smokeless (and black) powder, maybe under under rare circumstances a squib (far too light) powder charge may cause detonation which is otherwise known as a Secondary Explosive Effect?
Detonation may occur when using a squib load which is a serious concern when loading some light powder charge revolver ammo. I have not encountered low pressure shotgun data which produces much less than 6000 PSI in a 12 gauge. Okay, there may be a load or two at 5000 PSI but they are few and far betweeen...
To cause a detonation... If one would try to develop loads at much lower pressure than 5000 PSI they may cause a detonation which might burst or bulge a barrel at the weakest point between the chamber and the base of the wad. Perhaps a shock wave may result when the powder is mostly burned and the wad is still traveling down the barrel creating a vaccumn?
Another theory is that a reduced powder charge might be ignited too fast by the primer. If burn rate is determiend by the surface area of a powder then direct explosure of too much surface are at the energy of a primer might induce a faster burn rate? Just a theory of mine...
Where are the explosives experts when I need them?
That's my best for now...
Mark