Rereading your post, I see that I missed something on the first reading -- allusion to muzzle-loaders ("halitosis guns," I call 'em, for their muzzle odors).
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<br>Old muzzle-loader barrels were usually (I think always) iron, not steel.
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<br>Many modern muzzle-loader barrels have turned-out to be rolled tubing, not drilled rods or bars. The makers obviously assumed that they'd never be loaded to the higher pressures that smokeless powders develop.
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<br>Neither type can take much pressure.
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<br>Also, the muzzle-loader breech, even on a modern in-line halitosis gun, doesn't provide as good a gas seal as a brass cartridge case provides (another stringent limit on the level of pressure that's safe in that kind of breech).
