Slow or so-called "Progressive" powders were "Not" originally designed for the purpose of producing "Low" pressures,
You of course realize if you push a given wt of shot to a given velocity, you do not really "Drop" overall pressure, only re-distribute it. "IF" you push for instance 1oz of shot to 1200fps with two loads, one at 5K & the other at 7K, the 7K load will obviously have the highest "max chamber pressure". Not so obvious, the 5K load will have a "Higher Pressure" for the rest of the bbl. It is worth noting I think that bbl walls are "Always" thicker over the chambers than elsewhere, this was for a reason. Many will look at those 5K & 7K figures & think they are getting something for nothing, but "There Ain't No Free Lunch". What you "Save" at one point you "Pay For" at another.
My Sherman Bell articles are currently out on loan. However, I recall that in at least some of his "finding out for myself" tests, he placed strain gauges at several points along the barrel. I don't recall the phenomenon of higher pressure farther down the barrel with loads that generate lower chamber pressure, but I could well be misremembering.
