I want to break a kid's nerf gun
Why would you need to damage their property?
He keeps running around and shooting it and one hit me in the head.
I understand why you're annoyed at the kid, but breaking his Nerf gun isn't the right thing to do. You should ask him to be more careful, or talk to his parents so they can let him know that what he's doing is annoying. Breaking someone else's property is wrong.
You don't understand, I have to scare him.
I think that you will get further with him by just asking him to stop. I do not think that it is okay to break things that do not belong to you.