A little on the bigger size for a keychain, but quite small for a minigun! The barrels each hold a 6mm plastic BB and they can be rotated after each shot for quick shooting action! It's a very satisfying fidget toy as well... Assembly: You'll need superglue, a small 5mm diameter spring 15-25mm long and M3 bolt at least 10mm long. Watch the assembly video here: https://youtu.be/w5Dx0F3XRfo