I have been unable to find any laws this specific a subject involving guns. What I know is that i need a license to fire a gun that shoots a bullet or other projectiles but as what i am going to make is railgun, an electric launcher that wont actually fire a projectile but can still be used with bullets or any kind of small rocket. I dont want to start building until im sure and of course i wont be using it near anyone of any buildings as its very dangerous.