Hammer grip curls
The brachialis is the secret weapon of the biceps. If you train it correctly, you can achieve huge hands. The most effective biceps exercises in the gym to help you achieve this bump between the heads of the biceps and triceps are hammer curls or neutral grip pull-ups (when the palms are facing each other).
Doing a biceps curl in the gym with supination also engages the brachialis, which also helps build bigger arms. By training in this way, you will not only increase the volume of your arms but also protect your shoulders due to balanced muscle development and strengthening of the biceps tendon. Do not get carried away with training only one grip and work out all parts of the biceps.