Inazuma has creative ways to obtain the diagrams for the new Inazuma weapons. Here’s how to obtain the Hamayumi bow recipe from one of the three common chests behind Takeshi, the seashell collector. He can be found to the right of Nazuchi Beach as shown below:
Talk to Takeshi as he’ll tell you where to find the seashells which are along the coast of the beach. There are several spots such as the one below:
The spots will give you a random item ranging from ingredients, electro crystalflies, crab, enemies like a large hydro slime, and the Mysterious Conch needed for the chance at getting the bow recipe. Talk to Takeshi for the next seven days when you have three Mysterious Conches each day to open one of the common chests.
Note that the chests will give randomized items such as ore, and ingredients like apples or radishes instead of the bow recipe on the six days leading up to the seventh. After the seventh day, you’ll be able to obtain the Hamayumi bow recipe after opening one of the chests.
Once you obtain the bow recipe you also earn the achievement: Thank You, Come Again which rewards you with 5 primogems.