Well, she was researching poisons and building up resistances to them. She read about a guy giving himself poisons and venoms, steadily increasing the dosage and strength, until they had little effect on him. She wondered if she could do something similar but with bullets. She thought about shooting herself everyday, steadily increasing the size and muzzle velocity of the bullet, but Fuuka-san talked her out of that. Instead, she decided to start eating bullets and metal bits in an attempt to absorb the material..kind of like internal armor...
