Ballast is correct, a supinated chin-up will work the bicep better than your standard wide-grip pull-up. And, it will also work the brachials and various forearm muscles.
Most bodybuilders concentrate on the biceps when training arms and neglect the triceps. What many people forget (or don't realize) is that the tricep is the biggest muscle group in the arm. It is the tricep that makes an arm look big. So, do dips, close-grip push-ups and kickbacks along with bicep exercises for those massive guns!