You are right - the books say to use their data exactly as published. You will note the loads that produce usable (for ball) velocties are developed with the slower burning powders. There are no 1,350fps loads listed with 700X, Red Dot or other fast burners with 1 1/8 oz. balls. A 1 ounce ball would be about right for a modified choke 16 bore - way too small for a 12. Fast powders are used in the 1,000fps to 1,200fps trap and skeet loads. They are also the loads that develope maximum 12 bore presssures and must be used exactly as the book says.
; The loads and powders I referred to, generally produce much lower pressures and are the higher velocity shot loads using 1 1/8 and 1 1/4 ounce of shot. The powders I mentioned I'd used were 7625, 4756 and Herco to name 3. Blue dot and 571 are also powders that can be used, along with Hodgdon's longshot, although I haven't tried it. If someone locally stocked it, I would - it shows shot loads to 1,500fps.
; Now, what I've found is these higher velocity shot loads, when substituted with a round ball and the fiber and plastic wads to get the right column height, actually still develop less pressure than when shot is used. This stands to reason, due to the length of the 'bearing' surface of the shot, which creates friction. The ball only has the length of the plastci cas check bearing, hardly any 'scrub' at all, is there - at least, that's one theory for less pressure. Also, if you have Lyman's shotshell handbooks #2 and newer, you'll see their loads for .690" round balls, to 575gr. slugs are considerably higher than listed for like weighed shot loads. Also, they use a variety of fibre, card and gas-check-type wads. The PGS gas seal comes to mind - not available here - make my own from free trap wads.
; I hope this helps explain. If any further questions, don't hesitate to ask. There are no secrets.