Which bicep head is the peak?
The biceps are composed of two heads: the outer (long) head and the inner (short) head. The peak is formed by the outer biceps head; therefore, if you want to bring up your biceps peak you need to build up that head.
Do hammer curls work short head?
While hammer curls activate mostly the long head of the bicep, a traditional bicep curl activates the shorter head of the bicep. The shorter bicep head is what produces more of the muscle peak you might be looking for when trying to fill out those t-shirts.