"Think about it. 150 years ago the barrels were made of cast iron or low quality (by todays standards) steel. Many of the mountain men would load heavy loads for bear, loads that we would call magnum or hotter today"

Exactly! I have a book and the guy was doing testings on the hawken rifles and he was using more charge than todays 150 grain mag load. Im looking at it now and he was shooting a .53 hawken, prb, 187grains ffg powder. I personally have used more than 150 grains powder in my winchester just to see how it shot and i stopped at 170 grains pyrodex RS. I even have a video of Roger Raglin saying he is shooting a " i forget what bullet weight"700ish grain slug with 200 grains of pyrodex. He was hunting cape buffalo.