When I was a kid around 5 years old, several of the neighborhood boys had made zip guns. They were typically only around 10 years old.
They weren’t doing it to evade laws, they were just doing it because they could. I have no idea how to make one but it must not be that hard.
I would use the antenna of a be bug. And a .22 round. That’s it.
Used section of car antenna for barrel.Nail for firing pin sand rubber band powered.