In a pitcher, combine 6 ounces dark rum and 1/4 cup fresh lime juice; refrigerate. Meanwhile, place 1/2 cup sugar in a shallow bowl and 2 tablespoons lime juice in a saucer. Sugar the rims of 8 glasses by dipping them first in the lime juice, then in the sugar. Refrigerate the glasses. Add two and a half 12-ounce bottles chilled ginger beer to the chilled rum-lime mixture and divide among the prepared glasses. Garnish each with a lime wedge.