I agree with the genetics thing to an extent, but without question, any curl cambered will work the peak. do more hammer curls and stop using supination and you should see some peak action take effect.
Cambered and hammer curls work the outer head while supinated or flat curls work the bulk of the belly and inner head.
on your last set always do descending sets. keep pulling a plate off each side as quickly as possible and do some more reps to failure, then pull off again and again and again untill your down to barely any weight at all. no rest between everytime you pull a plate off. your arms should feel like razor blades slicing through every part of your arm.
Scaggs