Hello Keith,
Sorry to be late in this discussion. Yes, cast iron tubes did have trouble with the vents wearing out. Erosion occurred in the vents of the larger rifles more than smoothbores. It was a combination of higher pressure and vent erosion in iron rifles more so then smoothbores and bronze tube material. The vents were fixed by inserting a metal sleeve of mainly of pure wrought , some of wrought iron. This new metal sleeve had a vent in it and was replaceable. The repair process was called Bushing, or bouche.
Hope this helps.
Ron
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