For shots, you need to have strong Latissimus Dorsi muscles. You biceps also act as a negative force against it, so you would want to work on that too.
Having strong wrists and snapping them when you shoot adds power as well.
Since shooting should not be an upper body motion, it's more the core strength. Power for shots comes from your ability to rotate your hips & nips fast, and your upper body and wrists just provide the aiming and extra juice.
Work on your back muscles especially, lats are very important.