"Normally" if you use powders having different burn rates with the same amount of shot to the same velocity you will get lower pressure with the slower powder. If you load them to the same pressure you would normally have a higher velocity with the slower powder. Although it will only be a slight amount the slower powder would also shift the point of max pressure a bit down the bbl. "Peak" pressure is normally measured at 1" from the breech to insure the max is always recorded. Even if the shift is only 1/16" the bbl wall is now a bit thinner.
By careful choice of powders you could load 1Ľoz of shot to 1300 fps & 1oz to 1200fps & have the same identical "Peak" pressure. If you think the heavier load doesn't put more stress on the gun, then you have a total failure to grasp internal ballistics.
