So you want to build your own powerful mini gun? You may want to give this Mini Magnet Gun project by ValvexFTW a look. It is a magnet dart gun that can shoot magnetic ammo into wood, cardboard, and anything else you can think of. The ammunition is made out of a nail and attached to a small neodymium magnet when loading.
As the maker explains, even though this is a tiny magnet gun, you need to be careful how to use it. The above video shows what you can do with it.