barrels back then were low grade steel. Very, very weak. Every time someone tried making thin steel barrels, well, eventually a flaw would cause the things to blow out. If you have a good enough weak spot in a barrel, let the thing pit and rust, and there's only maybe .25 inches of metal, it will blow out at black powder pressures.
consider a couple things. with a percussion rifle, all that pressure is held in by only a layer of foil. with a flint lock, you actually have an open hole into the barrel, and no seal at all. this should tell you that the pressures are really pretty modest.
When you look at some of the guns made today, will see that barrels today have less than half of the steel used in BP days, and they even taper.
Dixie gun works had a gimmick. they would drill a flash hole in a barrel, screw plugs into each end after packing it with powder, and set the thing off with cannon fuse.
Golly, that barrel never blew up.
BP barrels were overbuilt in the old days because steel was horrible. Overbuilding saved lives. We did not change the process as the materials improved.
that simple.