DE of BP plus shot load is a "sorta" surrigate for pressure. Also, Brit loads tended to go to longer cases with increasing shot load. When you consider DE and case length tied to shot load, then shot load becomes an indicator of the pressure and power of the gun. Keep in mind that pressure measuring was a rare event until recently. Shotgun proofing has its roots before pressure was well understood and easily measured. Now days, shot load is one indicator of recoil - important to gun wood and the shooter.
