"if max pressure is around 7" from the breeches..."
Is that a typo ed? and a shotgun "breech" is not plural.

Every published pressure-distance curve, including the one I posted, shows the peak pressure to be 1-2" from the breech, even with 'Progressive Burning' DuPont Oval.

[Linked Image from photos.smugmug.com]

[Linked Image from photos.smugmug.com]