"Progressive" burning powder is essentially another word for "Slower" burning. This allows a heavier charge of powder to be used to give a higher velocity or push a heavier shot load (or both) without an increase in max breech pressure. This will be compensated for by a slight increase in pressure for the rest of the length of the bbl. Consequently I would expect the loss to be greater for the progressive powder load than for one loaded with "Quick" powder as would have been used in those tests Thomas reported on.
Thus if one is shooting max, magnum type loads they should expect a slightly higher percentage of loss in going to a shorter bbl than when using X-Mart economy loads with light shot charges & fast powders. In neither case is the loss going to be so great as to be of real consequence.
