Dave,
I own the clawhammer 10 gallon still:-
They use 16 ounce for the main boiler, boiler bottom, vapor cone, collar, everything except for the cap plate. The cap plate is the piece that accepts the Column and is thicker- I am guessing it is 14 or 15 ounce copper.
If you are building anything larger than 10 gallons I would suggest using thicker copper-