I agree that the difference between "trapdoor" and "modern" firearms is a very important split. And same with the quality of modern firearms as you mention. I try and push my .454 as hard as possible, while staying safe, and looking at the difference of starting and hot loads between powder manufacturers loads, average work up loads from myself and others I have seen posted, and sources like FA arms, which guns can handle the hottest around.
There are a lot of guns out there that with the availability of hot ammo as there is nowadays they should already be wallhangers - unless they are in the hands of somebody who has experience loading for the gun and knows its limitations.
It's a hard lesson as you have gone through to see something marked "44 mag" that was obviously built by someone who understans dimensions, but not pressure.