Apparently I don't understand internal barrel ballistics
A pressure - time curve with those same loads
A distance of the ejecta - time curve
Peak pressure with e3 is about .0003 sec. at which point
the ejecta has just started to move and has not yet "slammed" (did you miss the
?) into the forcing cone - at .0005 sec. - when the pressure is now falling??
I clearly need the help of a ballistic engineer to understand the contribution of the ejecta entering the forcing cone to peak pressure. I'll look around Neil's site
www.claytargettesting.com toward the bottom here are some links about "chamberless shotguns"
http://www.trapshooters.com/threads/model-12-forcing-cone.286074/ 
