it's a lot of things but it's savvy political move from her
1) she is a woman, she doesn't want to seen as weak
2) Democrats need better messaging on more 'male' issues. For too long most of the leadership has looked kinda like academic goofball professional class people, which only appeals to a certain demo
3) She is a former DA, lots of DA's have guns and know guns. Harris is no sheltered, ineffectual Harvard intellectual drama club yuppie type. She was a line prosecutor in Bay Area. She's seen some things.