The knowledge and skill in the HPR (high power rocketry) hobby is definitely there to build basic weapon systems. Active stabilization using movable fins is a thing, onboard gps and flight controllers are also a thing. Nobody puts it together end to end though because it gives the whole hobby a bad look and the hobby governing organizations strongly discourage it too. Also, fortunately, most people have no interest in mass murder either. Much of the hobby is old engineers who are retired but still want to work on engineering projects in their garage.