A guy can drive 10d nails with a sledge hammer. It is really nice to have that sledge hammer if on occassion you had to drive a railroad spike. It would be hard to drive a railroad spike with a 20 oz hammer so you better get a sledge hammer for the once or twice you'll drive railroad spike. A sledge hammer will be kind of sucky for the 100's of 10d nails you'll drive but, it will work.
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