Dents are like "handling marks" in wood: the result of mishandling. Takes a bit more mishandling, however, to dent a barrel. With a really fast swing, I suppose it's possible to dent very thin barrels by whacking them into a tree, as a grouse or woodcock hunter might. But most of the time, it probably takes an encounter with something even harder. Dropping the gun on a rock might well do it.