In Fishing Master, Bullet is the key consumable asset that you spend on fishing, and is also the revenue you get by fishing. You must deposit FMC to refill Bullets in your inventory in order to continue if it is fired up. You can convert your Bullets back to FMC everyday too.
When spending the bullets on fishing, you are also accumulating shells. Currently 1 bullet will automatically turn to 1 shell when you open fire.