In this video, I'll be showing how to make a mini spud gun that shoots BBs over 50 feet. You could also use anything else that'll fit in the barrel as ammunition.
This miniature BB gun is fairly easy to make, but you should be careful when firing it. I added tape around the chamber to prevent the bottle I used from exploding if it's not strong enough by itself to support the pressure when firing.
For the materials, I used a plastic supplement bottle (you can use any tough bottle), a barbecue igniter, BBs, two straws, and duct tape.
I tested out the red-dot sight and it was surprisingly accurate when I was using the copper BBs for ammo.