"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"

I've read books that also state this, but in addition to better barrels we'd also have to assume that the quality of powder is much purer (or better) than it was a 150yrs ago. I'd say just based on this assumption that our mag load of 150 would prolly be just about equal to the heavy loads the mountain men used. Not to mention that most mountain men prolly used guns that .54 caliber and above which can handle heavier loads and keep lower pressure in their barrels with heavier bulletsthan a .45 or.50 caliber(which are the two most popular inline calibers). stick to the manufacturers specs. be safe. Those are my thoughts.