That's a wonderful thing.
As has been very correctly said, welded steel barrels aren't even remotely solid, and they did have a certain number of pores. Pounding the steel into a solid welded tube over a smooth, are mandrel prevents some of the imperfections, but it's still just a bandaid for the inner surface
Inmates, sulfur compounds, and soot were pounded into every pore with every round, and cleaning out gunk from microscopic crevices can't be done. After a century, even well cared for, it's conceivable that some impeccable may have gone as far as putting serious defects almost all the way through the steel, and a weak spot in a weld might have an entire section just unravel.