Pressure is pressure. It does not matter what the source of it is--BP or nitro. Both attain their peak pressures in the chamber and quickly diminish in the bore. I have been surprised at how similar the pressure curves are between the two. My personal conclusion is a nitro load generating pressure equal to BP is gentler on the gun than BP.
I think there are good reasons American guns were built heavier in the breech than English or European. Americans tended to reload their ammunition and in general there was little understanding of the forces nitro was capable of creating. Some of the earliest nitros were designed to be bulk measured exactly like BP. Technology moved very quickly and soon nitro products had to be weighed and here a problem arose. Many continued to bulk measure it--often with disastrous consequences. Nonetheless, American makers tended to build heavier breech areas to withstand ungodly pressures. In contrast English and European shooters seldom reloaded and ammunition was generally standardized with acceptable and defined pressures. Therefore, gunmakers could safely build guns with lighter breeches. (Much of this is paraphrased from writings of W.W. Greener)
With today's reloading information and components it is very easy to assemble loads with pressures no higher than BP and produce velocities in excess of 1200 fps.
