Just my take on this subject & I know in advance many will dis-agree. Burn rate of shotgun propellents are controlled primarily by two means; Configuration of its granules &/or chemical coatings which "Retard" its burning.
Slow or so-called "Progressive" powders were "Not" originally designed for the purpose of producing "Low" pressures, but for pushing larger shot wts to normal or higher velocities without exceeding the max allowable pressure range.
Faster powders will therefore burn more reliably under low pressures & under a greater variation of temperatures than will slower powders. Most of the vintage guns with which we deal, will, if in sound condition quite readily handle loads producing pressures in the 7-8K psi range & were actually designed for loads up to about 9K psi. My personal preference is to use the lighter shot charge wts for the gauge, keep my velocities no higher than 1200 fps, usually around 1100-1150fps & to then use the fastest powder of my preferred brand which will give pressures in the 7,000-7,500 psi range. A gun which I don't figure capable of hadling this I don't shoot.
"IF" I were loading for very high volume shooting in warmer weather "Only" I "Might" use a load which dropped the pressures to about 6K, but no lower.
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.
