Short answer is Yes, you can use a dented drum. Do take a mallet and dent it outwards.
It is recomended, to use a drum with a removable lid. But not everybody can find one. Use what you have. One of the beauty's of cob is how easy it is to rebuild / make changes... just crumble your cob up and rehydrate! So later if a barrel with removable lid comes your way it will be easy to install.
Not all who wander are lost... J.R.R. Tolkien
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