It's fine to teach girls about guns - what is not fine is believing that is the solution to the problem of violence in our world, and violence against women, in particular. The number one killer of children in the US right now is guns. It's clearly not the answer to violence.
Rapist, murderers and other types of criminals that are the most likely to hurt a woman are the ones in her own home. Somewhere between 35 and 50% of all women killed in America are murdered by their current or former domestic partners. Women who fight back against violent husbands get the book thrown at them and end up in jail.
85% of rapes are committed by a man the woman knows and invited into her life.
The only real solution is to decouple aggression, domination, and abusive control of women from mainstream masculine values. Period. Full stop. Lots of other cultures consider violence to be lazy and rape to be unmanly. We need to move in that same direction, because "traditional" masculine norms aren't just harming women, they are literally killing men.
As the batterer-intervention counselor Lundy Bancroft observes in his deeply insightful book Why Does He Do That? Inside the Minds of Angry and Controlling Men, many men who batter have internalized cultural beliefs about manhood that legitimize — in their own minds — their controlling and abusive behaviors. These beliefs did not appear out of thin air. These men are not from some other planet.
Katz, Jackson. The Macho Paradox (p. 33). Sourcebooks. Kindle Edition.