I got the idea from someone else on here but their design was meant to be a handgun. My design is meant more to be a hand cannon though.
Step 1: Gather All the Components
27 yellow or gray 5 slot connectors
One orange piece
3 long gray or black rods
2 small bungee cords or rubber bands
Step 2: Making the Barrel
This part is easy all you need to do is put the last 2 connectors of the yellow pieces into two of the gray rods.
Step 3: Make the Inside
All you need to do is tape the orange piece to the remaining gray rod. Then put that into the whole in the yellow connectors
Step 4: Attaching the Bungee Chords
This part is quite easy just loop the bungee around the orange piece and then from there down to the bottom of the barrel. Tie one of your bungee chords 6 times in the middle to shorten it up a bit.
Step 5: Loading the Gun
You take a small white piece or a small gray piece pull the orange thing back put the piece in and then let go.