What you need:
1. A spoon - older ones are better because the contain ferrous materials, stainless steel doesn't do the job.
2. Neodymium magnets, one bigger and two smaller ones
3. A projectile that is ferrite free (wood, plastic, glass, rubber will do)
Works weakly with just one big magnet and a spoon but you will get a lot more power when the two smaller magnets are attached to the spoon - magnet pulling another magnet gives more power than just magnet pulling steel. Neodymium magnets are very fragile so for this it's good to have steel casing in magnets so that they won't break easily smashing together. These disc shaped magnets will do the deed nicely - block magnets would be more appropriate shaped but then again there aren't that many block magnets with steel casing to my knowledge. Any non-magnetic material will work as projectile. I used a marble, but plastic, rubber, wood, or rock will do also. Be super careful when you handle powerful magnets like this. The pulling forces of these magnets are 100 kg, 26 kg and 8 kg.
Step 1: Place the Smaller Magnets and Spoon on a 90 Degree Angle Over the Big Magnet
Move the spoon into position. it needs stick to the big magnet on a 90 degree angle so that it can easily be launched by pressing the spoon downwards. If you want to go pro you can even attach a trigger wire to the end of the spoon. I did it when I shot the gong in the video.. It's fun project for sure and there is lots of room for improvement this was just prototype.